Earlier this month, NSA employee Ms. Reality Winner was found distributing a printed copy of sensitive information to the publication Intercept. Although the NSA did not verify the exact means by which they were able to determine that it was Ms. Winner who printed the document, one possible way is via micro dots.
The letter you recently printed from your HP printer may well have a code on it, not visible to the naked eye. The code is comprised of yellow dots each less than a millimeter in diameter whose configuration against the (usually) white paper identifies the day and time the copy was made and also the serial number of the printer used.
This yellow dot code is a form of steganography, first developed in Japan. When printers were newly on the market there was concern that they would be used in forgery and particularly to counterfeit money. These codes would be a means of countering such crime. In a similar way, printers are able to detect a pattern known as the EURion constellation which is incorporated in banknote design and lets the printer know not to copy it.
Privacy advocates are against the yellow dot technology citing the potential for tracking down whistleblowers with legitimate claims or important information; reducing their ability to remain anonymous. The Electronic Frontier Foundation (EFF) is in this camp. “Individuals using printers to create political pamphlets, organize legal protest activities, or even discuss private medical conditions or sensitive personal topics can be identified by the government with no legal process, no judicial oversight, and no notice to the person spied upon.”
Others see this coding as a useful investigative tool that comes into play only once a crime has been committed. If you aren’t committing a crime, why would you care if there’s an almost invisible code on your printed copy?