1. There's lack of probable cause to say that anyone who just founds out a wallet with that money is guilty of breaking a law.
2. I guess so, who believes about "finders, keepers"?
3. That's actually it, if he knows the private keys of it and the situation really lacks of sufficient cause to say whether he technically owned it or not.
4. Yes.
5. Yes.

You can check UTXOs and find dormant ones but there are different criteria for identification of dormant UTXOs. With different thresholds, you will find different dormant UTXOs.

For example you can use this Bitcoin hodl waves with different bands of UTXOs and find your own-defined dormant ones.
https://www.lookintobitcoin.com/charts/hodl-waves/

More important, being dormant does not mean bitcoins stored in such dormant UTXOs are actually lost.
Most important, if those bitcoins are actually lost by owners, it does not mean you have rights to claim that those coins are yours, it's illegal and immoral.

I've read different opinions here from people about what they'll do if they find themselves in the situation captured by the Op, they're all valid.

However, I think we sometimes imagine stories around things that though are theoretically possible, but practically unrealistic.

Yes I know that it's possible for a random person to discover the private keys to a billion dollars but what are the actual chances that such thing will ever happen?

A wallet with digital asset of such value will not be kept in places were the private keys can be randomly discovered.

If you're trying to guess or gain access to someone else's wallet, even if you don't get anything from it, can still be considered stealing.

But whether it's intentional or not, if you happened to find wallet that has nothing to do with you and realized it had a huge amount of Bitcoin in it, what would you do?

Tbh, I find it really hard to believe that any of us could resist that temptation, even knowing it's stealing and you could even get prosecuted if you got caught.

If you can guess a recovery seed in your head and import into a wallet then you landed on a billion dollars balance then the security of the wallet is whacked job because with such amount of money they should do more than just recovery seed.

Such crypto wallet should have passphrase implemented, I should have landed on a empty wallet balance instead of this much amount of money, I will teach the person a small lesson my moving the funds into a new address then sending it back later.

Maybe after a week? Or two? I know such person would have shit their pants already,  enough time to atleast make them reconsider their strategy, it's going to be hard to restrict such temptation though, many will take some part and send the rest back.

It's certainly not legally fine but it's not a big concern at all because there is no working way to find Bitcoin private keys from dormant UTXOs and public addresses.

This is a challenging task that is impossible to solve at the present and even in the future, so legally accepted or illegally, who actually care because we all know that it can not be done.

Nobody will go to jail, not because stealing bitcoins of the other people is legally allowed, but because there will be no one can find Bitcoin private keys of the others simply from Bitcoin public addresses identified from dormant UTXOs.

Would it be okay if it happened that I discovered a bug in a software firm's wallet and I actually on that vulnerable and moved thousands of bitcoins to my custody as a new owner of the bitcoins. Does that give me the right to claim ownership just because I discovered a bug and technically didn't hack the system? Because from what am looking at it's no difference with gaining access to a bitcoin wallet through a guesswork of the wallet keys which happened to be correctly.

That a person mistakenly forgot his luggage at the airport because he was carring too many luggages, and then I was able to guess right at my first glance of it that that's a lost luggage that may have being forgotten by the real owner. It doesn't make whatever it's inside that luggage to automatically be mine. The law would not be emotional or passionate about the fact that it wasn't through robbery or any form of violence I was able to lay hold of it

It's not yours. As far as I know, no country allows people to take what is not theirs. If you somehow gain access to someone else's Bitcoin by guessing the seed phrase, using it means you're stealing.
It's true that if owns the keys, you control the bitcoin, but that still does not make it yours. Hackers that find a way to get hold of people's seed phrase and control the Bitcoin are considered as thieves, even though they have the seed phrase.
Nope. It's not his to own. Whether the person is dead or alive, the bottom line is it's not his to own. A person being dead doesn't give you the right to take their stuff. Even the bank does not have the right to take a dead person's money.

I understand so many persons will do the same thing but I want you to enlighten me on something. You said you will be returning it if it’s in hundred, just curious, how do you do that, how possible is that and also what if the person is death?

Whether someone guessed the seed phrase and got it right or saw it somewhere that it was hiding, as far as you are not the one that owns the wallet, the Bitcoin is not for you.It is true technically that anyone who is holding the seed phrase and can access and control the wallet is the owner, but in your own mind, it is not for you. So, that means it is not for you. And if you try spending it out to another wallet and you are caught, you are going to have to explain yourself and there may not be enough evidence to back up your claim.

In fact, if you even guessed the seed phrase and got it correct, that means you are a thief because, if you had no intention of stealing, you would never think of guessing someone else seed phrase. There is no reason why someone would want to guess another person seed phrase or password if their intention is not to access the wallet without the owner permission and steal,and doing that is totally a criminal offence because of someone claimed it guessed it. The question would be, why were you trying to guess someone else seed phrase in the first place?if the intention is not to steal.