If you found a $100 at the library, would you pocket it or turn it in? What about at a diner or pub? Confess!
If I find money in a reasonably "identifiable" place -- library, store, etc., -- I find somewhere to turn it in because I figure there's a reasonable chance the owner might realize where they lost it and come back. In the street, in a mall hallway, etc., not bloody likely, so I keep it. My biggest windfall that way was $200.