Every job contract, term paper, and yard sale flyer you’ve printed at home may have something in common. According to Quartz, several big-name printer companies have been embedding hidden tracking codes in documents for decades.

The practice is no secret: It was first called to the public’s attention by an article that ran on PCWorld in 2004. That report revealed that Xerox embeds sequences of tiny yellow dots on every sheet of paper that passes through one of their copiers or laser printers. Known as printer steganography, the clusters are staggered about an inch apart across the entire page, but with each circle spanning a millimeter in size they’re impossible to detect without the proper equipment. If they were blown up and made darker you’d be able to see an encoded message containing the serial number and manufacturing code of the source printer.

Unless you use your printer for illicit purposes, this likely isn’t something you need to worry about. The codes are there to make it easier for the Secret Service to track counterfeiters. "It's a trail back to you, like a license plate," Peter Crean, a former senior research fellow at Xerox, told PCWorld.

With the advent of the color printer came a new way for felons to forge money and legal documents. Coded dots were first used to combat this problem in Japan, and they became standard in Xerox color printers sold in America in the mid-1980s.

Printer manufacturers aren’t obligated to implement the technology by law, so there’s a chance it's missing from your computer. There’s also no law forcing companies to be upfront about this hidden feature, so if your printer does have it, it may be difficult to tell. One way to check is with a blue LED light and a magnifying glass: If the yellow dots show up you can be sure that page is traceable back to your printer, whether you mail it across the country or staple it to a telephone pole.

[h/t Quartz]