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Photo Source www.eff.org Electronic Frontier Foundation

Electronic Frontier Foundation has discovered that many printers add an arrangement of yellow dots to each page printed that can allow the authorities to identify the time and date of printing and serial number of the device.

List of Printers which do or don't print tracking dots

Decoding Guide

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.
"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.