You could probably infer a lot without knowing who traded it to who.

If say

* person A gets 100 banknotes from the bank, gives 50 to person B,

* person B pays 25 to person C and 5 back to bank (say, taxes)

* person C pays 5 of them to the bank

You can infer with some probability who traded what to who.

Not really. A billion other things also could have happened, they just didn't. Also, when was the last time you deposited cash?

Many people don't, but businesses deposit excess cash daily.

Certainly, though, those same businesses keep a lot of cash in order to give change to other customers. But that cash will still likely end up back at a bank before too long.

If it only goes to banks when businesses deposit it, you can't tell anything about the intermediary users

Just because you can't imagine way of tracking it doesn't mean it isn't possible.

Offer one...

The person who made this point addressed this already:

> banknotes are only in circulation for one or two exchanges before coming back to a bank

No, you don't get the 100% whole picture, but you can mine a ton out of data points like "note 38573204 was given to John Doe via ATM on 2023-01-10, and returned to bank by Jane Smith (owner of ABC Widgets) on 2023-01-14".

Not really. Coincidentally, there is no database of that. Hmm, wonder why!