Earlier quoted context omitted.
There are only 39.8 billion US banknotes in circulation, it can easily tracked in a Postgres database, banknote numbers when it exits the ATM, banknote numbers when they are redeemed by the baker, or by your drug dealer of choice, banknotes are only in circulation for one or two exchanges before coming back to a bank, it’s very possible that they are tracked to identify curious loops in circulation. In fact, if they…
Please correct me if I’m missing something. You’re describing a map where the key is a note and the value is, a linked list of banks? I’m not clear what the point is that you’re trying to make.
There may be extra steps in between, but if you're tracking a transaction / relationship type which happens twice or more, you can expect that the person on the record is the person you're after or can point you at them.
But I don't know why they expect FBI to actually have it implemented. The system would have to be so widespread/common that we'd know if it existed. I mean, there's https://en.wikipedia.org/wiki/Currency_bill_tracking but that's for hobbyists. Otherwise banks may check for duplicate numbers... but I can't find any mention of more widespread tracking.