EFF, the Electronic Fronier Foundation, have claimed that they have broken a secret code which some laser printers print with every page.
EFF researchers have found that a pattern of yellow dots is encoded in every printout from certain model printers by Xerox, Canon, HP and others. The dots encode such information as the date and time as well as the serial number of the printer.
The EFF claims the US Secret Service have admitted that they encoded this information in a bid to track counterfeiters, but the EFF claims the technology "makes it easier for governments to find dissenters". Source