My friend Andy pointed me to Note Trace, a website one of his friends has setup as a point of proof.

notetrace.co.uk allows you to track the geographical location of your bank notes. Using the unique serial numbers on the notes, you can see the path the note takes and the shops/locations the transactions take place.

It’s a nice idea, but it has one flaw and that’s to trust other people to input the data for it to work.
It could be interesting to see how this progresses. It’s got a nice user interface and seems quite easy to use so we’ll see what happens!