Tracking Documents via Printers/Copiers
Apparently, many color laser printers and copiers encode information like the serial number and manufacturing code on each page that is printed. The dots used to encode this information are very small, all of them covering less than one-thousandths of a page, and yellow. You may be able to see these dots if you shine a blue LED light on a page and use a magnifier.
According to Peter Crean of Xerox, there is no way to turn this off.