[bamboozled]

It’s pretty hard to track serial numbers , you can easily wash the money” yourself ?

[inglor_cz]

Some ATMs likely already track serial numbers, so you know when the banknote left the bank's custody and who withdrew it, using which card.

You can also track serial numbers when businesses return daily cash collections to their bank.

Especially the biggest denominations won't be passed back to other customers in the meantime, but rather sent back to the bank early.