A research team led by the EFF said that it has broken the code behind tiny tracking dots that some color laser printers secretly hide in every document. "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 Seth David Schoen, staff technologist at EFF.
This is actually a great idea, unless your a kidnapper and need to use your printer for hostage reasons, then I think your pretty much screwed.
The tracking codes have been around or in use since about 1999 or earlier. Enjoy the Full Story here