On a related note: many colour printers leave a tiny encoded pattern on each page (based on model/serial/etc), ostensibly to help track counterfeiters. [1] [2]

Unfortunately, they're printed all the time.

[1] https://www.eff.org/pages/list-printers-which-do-or-do-not-d...

[2] https://www.eff.org/issues/printers