When I was 18, I worked at a supermarket in the dairy department, and I found a $100 bill on the floor. There are cameras all over the supermarket, so I put my foot on it, and slid it under something then picked it up. The entire shift, I kept thinking that I got set up by my managers and every time I saw one of them, I thought they were gonna say something. It was awesome finding a benjamin on the floor.
Another time, I was working on the cash register, and I stepped off to go get something, and I found a $50 bill on the other side of the register where they customer stands. Picked it up with quickness. Never thought to give it to customer service, because what the hell are they going to do with it? Keep it themselves.
Also, when I worked self checkout, people would always leave money behind in the dispenser and leave without it. If I noticed they forgot it, I would stop and tell them, but if they were already gone, that money was mine.