This site is a gigantic database, were the members give information about their euro banknotes (and coins), and then put them back into circulation. Also when you give the info about the banknote, you get to know how many simillar banknotes that have been posted. If you are lucky someone else get your banknote, or you have picked up another members. This has actually happened sometimes.
Here is the link: http://www.eurotracer.net/information/notes
This site has also interesting info about euro banknotes.