https://en.wikipedia.org/wiki/Printer_tracking_dots

Printer tracking dots, also known as printer steganography, DocuColor tracking dots, yellow dots, secret dots, or a machine identification code (MIC), is a digital watermark which many color laser printers and copiers produce on every printed page that identifies the specific device that was used to print the document. Developed by Xerox and Canon in the mid-1980s, the existence of these tracking codes became public only in 2004.