Most printers add a tiny yellow Machine Identification Code dots on every printed page. Makes it pretty simple to identify who purchased the printer.
https://en.wikipedia.org/wiki/Machine_Identification_Code
This is only true with color laser printers and copiers. If they used an ink-jet printer, or a B&W laser printer, then there is no identifying mark.
That’s why I buy my printers at yard sales.🤔