Tell us a story: found money

Have you ever found someone's lost money inside of something (say, in a bank envelope or a paper bag or a dropped greeting card)? Obviously, finding a $20 on the street by itself is very clear: you can't know who dropped it, so it's yours. But money inside of something can potentially be connected to the person who lost it. If you have ever been in that situation, what did you do? If you've never been in that situation, what would you do?