My crazy idea is constructing a chip with diminutive video-camera, microphone and a radio-beacon and attaching it to a $100 bank note involved in a business transaction.
This way all further financial operations this bank note would be involved in could be followed and registered electronically, revealing all the "vague paths" it may follow.
This technology could also be effectively used for monitoring drug, arms and human-trafficing, preventing and punishing theft, robbery and loss.
As long as such technologies are scarcely available this idea could be used at least as a plot for a movie