I've found wallets full of cash and credit cards. Always handed them in. As for keys, though it's tempting, and finders keepers is a thing... hmmm. No answer. Haven't been in that situation. Tough call. Technically, you're not in possession of the funds, only the keys. That would be theft. Not sure I want to be a thief. Then you have a live a lie, especially if it's a lot of money. I can't see how that could be fun.