I obviously can't speak for every country, but most (all?) states in the US can charge you with theft if you find and keep a significant amount of cash or anything of value and make no attempt to return it to the true owner or hand it over to law enforcement. This has never been tested against finding a paper wallet as far as I am aware, but I would suspect it would be a similar situation given that a paper wallet is obviously valuable. However, a paper wallet here is unique in that the contents could be stolen by anyone who sees the wallet at a later time and without leaving a trace, which will generally not be possible with cash which is handed over.

I still think the best thing for OP to do is simply post the address so the true owner can try to find him. For all we know they are hunting frantically for any mention of this address online.