For some reason, I've found quite a bit of money outside.
Back when I worked at Rush University Medical Center in Chicago, I once found ten $20 bills in a flower planter box on the edge of the campus. It was late at night, around midnight, and I had just left my office to walk home a couple of blocks away. I returned it to one of the hospital cops at my office building. Three months later, on the same day when I later got the money back (because no one had claimed it), I found a $5 bill on the public sidewalk on my way to work. I figured that was the interest on the original $200.
I've also found cash in parking lots and the like. I think the last time it happened, I found a $10 in front of a grocery store that I frequent. If I recall correctly, I checked with the store manager to see if anyone lost some cash and then pocketed it when he or she said no one had.
If I see a penny or a coin on a floor where it could be a slipping hazard, I'll pick it up just to remove the hazard. (I don't want anyone falling down at the hospital or at any of the stores I shop in!) If there's a place to leave it (like a donation box), I'll do that. Otherwise, I'll pocket it. At that same store where I found the $10, I'll sometimes leave nickels and dimes on the brick work on the columns in front of the store, just for the hell of it ( here's a picture of the store, in case you're interested in what the columns look like:
https://www.google.com/maps/uv?hl=en...IVDNFjCh0HmAIW ). I've gotten up to 1 bucks or more (a nickel or a dime at a time) on a column before it's noticed by anyone and removed.
Sometimes when I have a bunch of pennies, I'll leave it in the change return box for a pay phone or a vending machine, just for the hell of it. I imagine the person finding it thinking it's a bunch of dimes only to discover it's just some stupid pennies.