Developed in the 80s and used since 2004
a Machine Identification Code (MIC), tracking dots or secret dots, is a digital watermark that laser printers and copiers leave on every single printed page, allowing to identify the device with which a document was printed and giving clues to the originator.
Maybe tracking dots could be used to confirm the source printer after a suspect has been identified, but its highly doubtful the government can identify the suspect simply by reading secret tracking dots on a page of print.