Printer tracking dots
Printer tracking dots are rather secretive, but are undeniably inserted in printed documents on many current-day color laser printers, with rumors of the existence of secret agreements between governments and printer manufacturers to ensure this "feature" is present on all major printer models. They are hard-to-see patterns of light-colored (usually yellow) dots that can, to knowledgeable observers, allow it to be determined when and by whom something was printed. It has been reported that this played a role in uncovering the government leaks of Reality Winner (yes, that's her real name) in 2017.
Rumor has it that printers are transitioning toward using steganography instead of tracking dots, so don't assume that just because your printer doesn't print these dots, it is not tracking you.