Color printers include back door for Secret Service

Concerned that color laser printers could be used to produce counterfeit currency, the Secret Service secretly worked with printer manufacturers to include a code that identifies the date and time of every printout, as well as the printer's serial number, the Electronic Frontier Foundation reports. 

EFF worked with printouts from Xerox DocuColor printers to discover the meaning of a pattern of hard-to-see yellow dots produced by the printes. "We've found that the dots from at least one line of printers encode the date and time your document was printed, as well as the serial number of the printer," said EFF Staff Technologist Seth David Schoen. EFF provides details and a program for decoding your own printouts:

You can see the dots on color prints from machines made by Xerox, Canon, and other manufacturers (for a list of the printers we investigated so far, see: http://www.eff.org/Privacy/printers/list.php). The dots are yellow, less than one millimeter in diameter, and are typically repeated over each page of a document. In order to see the pattern, you need a blue light, a magnifying glass, or a microscope (for instructions on how to see the dots, see: http://www.eff.org/Privacy/printers/docucolor/).

The Secret Service denied the agency is interested in tracking users, according to the SF Chronicle. "It's strictly a countermeasure to prevent illegal activity specific to counterfeiting," agency spokesman Eric Zahren said. "It's to protect our currency and to protect people's hard-earned money."