what would you do if..

I think I would take it to the police, because something that's that important to a person, is likely to be reported missing or stolen and if a person is looking for it, they will be so happy that they can get it back, rather than always being told the police have no information because you decided to give it to a charity.

I would not keep it. I would bring it to the nearest police station in the hope that the person that lost it may inquire there. Where I live that is the usual way of doing things - so there is actual hope that the owner might get it back.