This "all colour printers add hidden tracking codes to your documents" has been known, if not widely I guess, for something on the order of 20 years. In the same way as "scanners won't scan currency", I guess. But: has anyone ever torn open a printer ROM and documented how the process works?

https://www.eff.org/pages/list-printers-which-do-or-do-not-display-tracking-dots