There’s an Invisible Tracking Code Embedded in the Documents You Print
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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.
https://mentalfloss.com/article/501936/theres-invisible-tracking-code-embedded-documents-you-print