Like the title says what are the rules. If I miraculously got a private key to a wallet that had Bitcoin in it but was not mine originally then does that mean I now own the wallet like finders keepers and is there any legality issues with this in the usa,uk,canada,south america. Is this more of a morality should you or should you not.
Such miracles don't happen, but I it does, you just got yourself free bitcoin, they is no way I trust anyone to return it and how would you even know the owner of the wallet.
Wallets are actually location on the blockchain where your bitcoin are stored and private keys are your access to the location on the block chain, I Don't think there is anyway, you would actually end up with someones private key even if you try to guess.
You didn't hack so I don't think any laws are against you on this.