Honestly, I don't know even if they track the numbers. I just know it's possible. Certainly they are used when cash is stolen. It's more effort than CBDC will be. This is why they're pushing so hard for central bank digital currencies. My guess is, if you've kept your notes for years, they may not have you associated with the numbers. You can always exchange yours with other private individuals so the feds can't find you directly. They'd have to do actual detective work. And government employees are not keen on working too hard.