I also vote for returning it. I'd want someone to return mine if I lost it.
Tangent: I found $100 (5x$20) cash on the floor of a supermarket once. Instead of turning it in to the office, I walked around to see if I could spot anyone searching for something. It ended up being a lady and her two kids and she was walking around frantically, eyes glued to the floor. I asked her if she lost something and she said yes, all the money she had for groceries, $100. I handed her the money (at the time, that was about two days' pay for me), and she burst out crying "Thank you, thank you so much!" That was about 20 years ago, and it still makes me feel good to remember it.
Harvey is right, the rush is worth it.