You approach an ATM. You see a 20$ bill in the slot! The woman who was there before, and rightfully owns the money, can hear you if you shout to her. What do you do?
I don't shout, but I would most definitely run after her and try to return it.
I *always* try to return money, or anything else someone has lost, if at all possible. It doesn't belong to me, and other people have done exactly that for me in the past - going out of their way to do it, in a manner that showed me it's simply the right thing to do.
There is no doubt in my mind that I would shout and even run after to return the money. It's called bring a decent human being.