If you found a $100 at the library, would you pocket it or turn it in? What about at a diner or pub? Confess!
It's funny this came up, because I actually did find 100euro on the ground in a car park a few weeks back.
A tenner, or even a twenty euro note I'd have possibly just pocketed it, but 100euro...
I was on the way out of the car park at the time, so I waited until I was heading back to the car to hand it in to a guy at the security desk. Whether he pocketed it or not, that's up to his conscience.