Your printed documents have fingerprints - Machine Identification Code (MIC)
In the 80’s Xerox designed a way to hide information in plain sight as a way to combat counterfeiting. An ingenious, simple, and hidden code of small yellow dots called Printer Steganography were added to each printed document to include the date and time of printing and the serial number of the printer.
This yellow dot matrix is virtually imperceptible to the human eye, but can be easily captured and decoded with a scanner or camera. Since then, new variations have been used, including using various shades of grey within the printed matter to include data. Whatever you print with, there’s a very good chance there is some form of MIC being included on your printouts. Is it ingenious tracking? Absolutely. Is it a good thing? You decide.
In short, remember that your printed documents include the ID of the printer from which they were originally printed.