Mysterious letter

This past week I received a letter with no return address on it. Inside was a plain paper folded over another paper, folded over a $20 bill. The inside paper said, "I wasn't able to pay you for something you did for me a long time ago. Thanks for your help!"
The timing was impeccable (as most things like that are) Bill's birthday was this week and I was able to contribute toward a large gift David was getting for him (A male bag - much better than a cardboard box that he normally carries his classes' papers in).
But I can't help but wonder who would have felt it necessary to pay me for something I had done, or what I had done to merit such a thought. I've said it before: it's hard to receive good things anonymously, then I have to be nice to everybody!