MIT's Media Lab has started a website that helps consumers contact the manufacturer of their printer so they can request that "tracking dots" be eliminated from their machines.
The dots hold information about the serial number, make and model of your printer and were placed there at the behest of the Secret Service -supposedly to help locate counterfeiters. Seeing Yellow, a project by MIT's Media Lab, claims that after contacting the printer manufacturer about how to disable the tracking dots, the Secret Service showed up at one consumer's door asking questions.
Most color laser printers made and sold today intentionally add invisible information to make it easier to determine where (and when) a particular document was printed. This seems to have been done as part of a secret deal between the United States Secret Service and the individual manufacturers. Some of the manufacturers have mentioned the existence of the tracking information in their documentation, and others haven't. None of them have explained exactly how it works or what information is conveyed. No law requires printer companies to help track printer users this way, and no law prevents them from stopping this practice or giving customers a solution to avoid being tracked.
This information is most famously known to be coded by patterns of yellow dots that the printers add to the background of all the pages they print. The yellow dots are hard to see with the naked eye, but can be seen under bright blue light or with a microscope. Their arrangement reveals which printer was used to print a particular document, and sometimes also shows when it was printed. Some of the codes have been understood while others are still mysterious, but none of the printer manufacturers has denied that the dots are intended to help track a particular document to a particular printer (or that they can actually be used for this purpose). This is a direct attack on the privacy of the owners and users of printers, and in particular, on their right to free, anonymous speech.