The point is that we don't know for sure if the original owner lost the keys completely or just one copy thereof. It's not like physical cash, which if unclaimed can be reasonably assumed to be lost after a certain period of time.
+1
If OP can properly set up a cold storage (maybe with Tails?) it would be much better to move the coins to a new address, so the owner has a better chance to notice something is wrong.
And after some more months, if no "proper owner" emerges (who can sign a new message, we all know the drill), the OP can see what he will do, even donate those bitcoins.