I found 20 bucks in an ATM at the mall once. I felt bad good luck trying to figure out who it belongs to. Go around asking everyone if they lost 20 bucks? I don't think that would really work and it would probably just fall in some dishonest person's hands anyway. So I kept it.
Funny thing is, I almost lost way more in an ATM at a convenience store after that. Karma? lol. I took out 100 bucks, grabbed the money, went to pay for something and left. When I got home I had like 40 and some change in my pocket, and was trying to figure out how I ended up with that little, retracing what I bought etc. Then realized something is clearly not right. So I logged in to my bank figuring maybe I took 60 out. Nope said 100. Drove back to the convenience store and went to check the ATM, sure enough there were two 20's stuck on the top part of the slot. I usually count the money when I take it out of the ATM, but that time I didn't for some reason.
If I found like 100 bucks, then I'd probably keep it, but keep an eye out in case someone posts something on FB about losing money or something then find a way to get it back to them. Suppose you can bring it to the cop shop, but pretty sure they'd just pocket it and call it a day.