What do you do with the random money you find laying around the library? I'm sure some people think they are paying their fines by leaving dollar bills in their books, but they might as well be expensive bookmarks.
Although I keep eyeing these
The easy way to deal with this? Just throw it in the donation box. The realistic way? Go to your people in charge and make a written policy that unclaimed cash will the donated to the library (or friends of the library, or staff room snack fund) There is no reason to leave cash laying around for sticky fingers or hoping patrons will come back to retrieve it. Of course, if you can figure out where the money came from by looking up the last patron, you should call, but offering to turn it into a donation might make the patron happier than thinking it's going to the grubby hands of some overpaid union employee (ha!).