How can you trace a loop unless it involves someone reading and reporting the serial number?

Do you have some more information to read about how paper money flows are traced?

This! This is so infuriating. Now i guess most major city have some poorly secured db that contains all the time/place points where every user of mass transit has checked in/out. I'm not exactly sure what is the strong push for this, is it lobbying by vendors to get access to this data? In any case most of the time it's marketed as somehow "green" (because you understand, replacing paper with microelectronics is really the way to go).