Basically documents printed on certain model colour printers have a dot code embedded into them that identify date, time, and the printers serial number. Why would they do this? Because it's a great way to fuck over counterfeiters. (and other criminal uses)
Here! give it a go:
A research team led by the EFF recently broke the code behind tiny tracking dots that some color laser printers secretly hide in every document.
The U.S. Secret Service admitted that the tracking information is part of a deal struck with selected color laser printer manufacturers, ostensibly to identify counterfeiters. However, the nature of the private information encoded in each document was not previously known.