The principle of Consent, that people must not force perceptions on each other, is where all concepts of property and ownership come from. This is a very simple rule, but the applications are nuanced and non-obvious enough that I'm going to go through them in as much detail as I can. Even I'm not certain that I have all the details of the system right, but these are what I'm pretty sure are the rules underlying my conscience.

Restitution and Retribution

As per the formal statement of the principle of consent in my moral system, intentionally violating someone else's consent forfeits your own to the same degree. Full restitution becomes wide open to being extracted by force. I should keep it clear that retribution and restitution are still fundamentally separate things. Normal crimes are subject both to forceful extraction of full restitution and to retribution. But it's possible to intentionally violate someone's consent while doing the right thing. For example if you do it strategically for a greater good. In such a case you owe full restitution to the best of your ability, but aren't subject to retribution, because you haven't sinned. An unintentional violation of someone else's consent doesn't forfeit your own, but it does require you to perform restitution at least up to the point where your loss is as great as theirs (and if you have the option to do this and don't then you do forfeit your own consent). The reason that it only goes halfway is because once you perform half-restitution, their loss has been cut in half, and so you've actually already sacrificed your own rights to fully the same extent - because they've gone back up as you've gone down. Intentional crimes go all the way because you knew what you were doing and chose it, which constitutes giving your consent, so you can't claim equal victim status. Whereas in the unintentional case, neither of you is at fault; both of you are victims once you perform restitution.

And yes, this does mean in cases where you have partial knowledge - such as if you do something that you think will cause someone else $10's worth of property damage but it ends up costing them $100's worth - then the first $10 must be fully restituted and the rest half-restituted.

In an uncertainty case, such as if you do something that you think has a 50% chance to cause X property damage, if nothing ends up happening then you don't owe restitution, although you've still sinned against both Compassion and Consent and can (barring some justification) be punished on the grounds of retribution. If the damage does end up happening, you owe an amount of restitution halfway between it being a total accident and being totally on purpose.

Bodily autonomy

Since people inevitably feel what happens to their body, one must not take actions that affect another person's body without their consent. Due to how directly this is felt, almost nothing can override this. It isn't limited to pain or injury either. It is a consent violation to force any type of perception onto someone, including tickling, tapping to get someone's attention if they've previously stated that they don't consent to this, or continuing to speak to them when they request to be left alone.

External property by possession

If nature has handed you possession of an object, whether apparently by chance or as the result of an action you took, other people must not do anything that prevents you from using it as you wish. This becomes clearly the same idea as bodily autonomy when you think about metaphysics: If both affect your experience, there isn't a difference of kind between your limbs and your property, only a difference of degree.

It doesn't have to be in your immediate physical control the whole time. If you find an unowned piece of matter and place it in your house or somewhere else where you'll be able to retrieve it, it remains your property because you haven't really lost possession of it, just stored it temporarily. If you drop it off a cliff, or do something else that prevents any foreseeable future of you getting it back, you'll have a Lost and Found scenario, see below.

But, "possession" is an iffy concept here, because there's a caveat:

No ownership of power

If you have fairly acquired an object that allows you to affect the lives of other people in another universe, but you don't actually get any feedback of any kind on what happens over there, then it's not your property. Someone else is not forcing a perception on you if they steal the object and use it differently than you would (we're assuming the object is totally worthless on every other account).

More generally, if you'll never experience any result of the "theft", then you can't have been wronged.

External property by investment

If you act in a way that creates a noticeable external change in the world, such as building something, cutting down a tree, or just moving an object that no one owned to a new position, then others must not prevent you from enjoying the result of your actions (provided the results aren't affecting them as well). To do so would be destroying your labor as far as you're concerned, and thus equivalent to forcing it on you. Note that this means the degree of infringement is equal to the degree of labor you invested, not the result you're deprived of (I'll get to that in a second).

This stacks with ownership by possession. Violating either one without the other (taking from someone what they possess by luck or collecting the benefits of their labor that they don't possess and not compensating them) is theft; violating both at once (taking from someone what they've created and possess) is double the gravity. (More precisely the gravity would be the value you take plus the labor you waste.) Although in practice it would probably be somewhere between 1 and 2 times because assets are partly interchangeable via currency and most people have some of their fortune due to luck and some due to hard work.

Shared property

A lot of relatively reasonable people deny the existence of this altogether on the basis that only individuals can own things.

But shared property is inevitable. Imagine two children home alone and some transient opportunity comes up and they have to make a decision about the property (or perhaps the parents die). Assuming the parents didn't plan for this and specify who gets to act in their stead, there's only one possibility: shared ownership (or moral conflict I guess but I doubt anyone wants to stand by that here).

Or suppose two adventurers kill a dragon together and find treasure in its lair that can't be split (such as a single gem that provides magical effects).

In situations like this, what you have to do is believe that any decision one of the owners doesn't want is a violation of their property rights. You always have to compensate the person who isn't getting their way with something they agree is fair. This should usually be easy due to transitive valuation, but if no agreement can be reached in time and a decision must be made, the co-owners could unanimously agree to entrust it to randomness or an outside party's arbitration, or engage in moral conflict. And then in either case compensate the loser(s) to the best extent possible without making it not worthwhile for the winner(s) to win.

Most often you can reach a peaceful arrangement here by alternating periods of time where you have complete ownership minus the right to sell the common property or anything else that would prevent you from giving it to your partner when it's their turn. Or perhaps alternating by amount of use (measured in whatever way applies). But such a deal of course has to be accepted unanimously by all co-owners.

Sharing nature

After realizing how external property by possession and shared property work, a realization - one that is disconcerting for capitalists - can be had: appopriation of natural resources forces an effect on those who could've had it otherwise. You can see this principle working less deniably with this example: your house is next to a river. Assume you didn't know about the river when you bought the house, but now value it highly. A company from another area sends in trucks that harvest all the water and leave none for you in the future. Even many anarcho-capitalists would agree this is theft, but what's the difference of kind between that and this:

Your hometown is destroyed by a natural disaster, and you move to the nearest other area that has a water source. The river there is monopolized by a company that uses machines to harvest all its water. They have taken resources which, otherwise, you would now have access to. Time can't be the differentiating factor because even the water taken from the river next to your house you weren't already using; they let you keep the water you had already extracted. Both cases only involved potential access to the natural resource, and yet we can't avoid the conclusion that it counted as you "possessing" it for the purposes of property.

Of course, it is also theft to take the water from the company who's now using it. They have at least as much claim by possession and a claim by investment that you didn't. But you do have some valid claim to the water, and so you are entitled to some amount of it. It's shared property to some degree.

This doesn't even harm the capitalist notion of property as much as it might seem, because it only applies to natural resources - something that wouldn't have been there without someone else's actions, you can't claim this with.

Lost and found

From the interaction of the above principles, a few scenarios can be recognized:

Favors and gifts

When you do something for someone else without negotiating repayment, you're consenting to not receiving any because you had the choice to not do this. If you do communicate that you're only doing this under the assumption of being repaid but nothing is mutually negotiated, then they must repay the minimum of your sacrifice and their benefit from it. That's because you didn't consent to doing it for free, so if they repay less than that they're violating your consent, but they also didn't consent to anything, so they can't be obliged to give up anything worth more to them than the result of your favor.

But if we're talking about a gift rather than a favor - not a single transaction of fortune but the transfer of control over an object - then you absolutely do not lose ownership. Consent is retractable as long as the event isn't sealed in the past. This should be thought of as an ongoing favor than can be stopped at any time even if no "repayment" (because that's not what's happening here) was negotiated.

Non-scarce goods (freeloading)

The situation that a software vendor makes their software available for download with pay and (against their intention) you have a way of downloading it for free can be exactly rewritten as follows:

Alice makes a substantial personal sacrifice to provide some benefit to Bob, telling him she's doing it under the assumption that he'll do something else in return, although Bob didn't actually get any input in the matter. This is exactly the inaltruistic favor scenario above and the answer is the same: Bob must repay her the minimum of the value of her sacrifice, his benefit, and her requested price (since she consented to that).

Of course, the specific nature of the software version of this scenario creates different abilities for the consumer. In particular there's nothing wrong with the consumer pirating it and paying after using it - he can't be obliged to pay more than the software is worth to him and paying before using it means paying before finding out how much it's worth to him.

The obvious difficulty might be: but surely it's always moral to not use the work and pay nothing, and if the only difference between a moral course of action and the one in question is that someone benefits, how can that be considered theft? But here's the insight: if the work isn't worth anything to you, then you wouldn't have been asking this question anyway since the value you're obliged to pay is zero. If the work is worth something to you, then by definition the alternative to freeloading would have been paying that amount to use it, not not using it.

Any other form of non-scarce good would be the same way: using someone's artwork, for example. It's worth noting that using someone's artistic or intellectual property without paying them but with credit and in a way that doesn't devalue the product they're selling (such as posting music from a movie on Youtube) is generally of substantial benefit to the artist, since it gives them publicity - and therefore more fans - at no cost to them. A lot of people who are stingy about copyright of their works overlook that and think they're being stolen from when they're actually being helped.

There is an edge case though. If it is impossible for the consumer to offer any sort of compensation (and it always will be to the best of your knowledge) then there is no legitimate reason for withholding the work. The creator gains no benefit from doing so and the consumer is acting blamelessly.

Trespassing isn't the crime you think it is

Privacy in one's home is 100% legitimate; entering someone's home without their permission is forcing your presence on them and in a particularly upsetting way at that. (It's also worth considering in the house case that just by opening the door you probably waste some money on air conditioning or heating or let bugs in.) However, since you can only violate someone's consent by causing them some unwanted experience, being on someone else's land but not in their house or in their immediate yard is generally not a crime because it won't cause them any disturbance, and insofar as it does it's probably such a tiny one that they're literally better off letting it slide than extracting all the restitution they can for your unintentional disturbance. If it does happen to cause a significant disturbance by some unfortunate combination of circumstances then you can make restitution. There's no need for any hostility because there's no sin.

This consequence may seem instinctively repulsive to you. Therefore, I'll give another intuitive reason why you should reconsider. Suppose you live in a small circle of land, less than an acre, and all the rest of the world is 'owned' by other people who don't allow anyone on their land, so you can't leave your area. You are therefore in prison, are you not? Where do the landowners get the right to keep you there? Even if imprisonment were a valid punishment (it's not), you aren't a criminal. Does your conscience really tell you that you wouldn't have the right to trespass?

Sub-ownership

Sub-ownership is when a person owns something, but gives control over it to someone else for the foreseeable future. The "child owner" has full ownership of it for the purposes of conflicts with third parties, but the "parent owner" still has full ownership over it when conflicted with the child owner.

Parent and child owner are accurate terms because that is the most common example of this. Children often have "property" or "belongings" that the other children have no right to interfere with, but the parents do, because it's still coming out of their pocket and the child isn't entitled to it.

Restitution after death

A common situation that I've heard a lot of people talk about but never elaborate on because they couldn't resolve it in a way that satisfied them (and it bothered me too for a while) is that A steals from B, then A and/or B dies. There are a few possible cases here:

Of course, all that is under the assumption that the criminal and victim will never meet again and so properly resolving the situation is impossible. But if we're not to assume that people stop existing when they die, if the criminal and victim were to meet after death in some other world, the debt and/or crime would still be there unless the criminal had been punished or redeemed themself and the victim restituted in the interim. If only one of those is the case, you'd have to balance for example a victim owed restitution from a criminal who does not owe restitution. The whole situation is messed up at that point because a third party got involved and now can't be called in for the resolution, but you can try to reach a compromise. The only reason I advocate resolving through the above methods is because from what we know a reunion between the criminal and victim in such a way that they'll recognize each other is very unlikely.

Misunderstandings

Misunderstandings are unfortunate but they happen. For example, if you labor on an object not knowing it's owned, you still own your investment, but the object also still needs to be the original owner's property. Time / order of events doesn't actually make a difference if you didn't know about it (I'll explain what does in a minute). Since both people have valid ownership claims, assuming it isn't possible to separate the improvements made from the original object, we go to shared property (see above).

Obviously, if you labor on someone else's property knowing it's someone else's property then you can't claim any ownership at all of the resulting object that conflicts with theirs. (The ideal solution is still to separate out your labor if possible.) If there was a probability of it being owned, you multiply the ownership ratio by that.

Lying is another act that's commonly defended from the same realm of moral judgement as obvious crimes like theft and assault. That's wrong. Giving someone false information generally affects them and insofar as it does, you're responsible for forcing that effect on them, since you gave them false information without their consent. Therefore, lying is to be considered a crime at the magnitude of the harm it causes (of course the liar's guilt depends on the harm they expected it to cause; but again redemption and restitution are separate and someone could owe more of either one than the other). (This doesn't mean it's okay to tell a 100% harmless lie; just that it's only against the value of Truth and not against Consent.)