The owner might not browse those internet boards or be much active on bitcoin-related discussion boards at all. You can take some funds out and return the same coins with a message, but the owner might not be looking at what is happening on the blockchain.
Maybe. But maybe he does. In any case, cash found on the street is not better in any way.
However, Bitcoin wallets I wouldn't expect to find in a normal wallet (physical wallet, that stores money, and cards), since I don't really associate it with that.
And I neither expect from people to carry paper wallets on their physical wallet. It doesn't make sense to me. Paper wallets are backups. You don't carry backups on your bag, unless you move away. If you want to spend bitcoin outside your place, use your mobile phone with a lightning wallet installed.