If you find something valuable that’s obviously been lost by someone else, and you keep it without trying to get it back to its rightful owner, you’re no better than a thief. You’re a greedy thief. At least make an effort to return it! You can post it on CL lost and found or on your local Facebook group, whatever. But just keeping it for yourself is a really shitty thing to do and that karma is going to bite you in the butt someday.