Printer Steganography

Here is an article from 2005.

AP - EFF Reveals Codes in Xerox Printers

Just because a document from a color laser printer doesn't carry your name doesn't mean no one can trace it back to you, privacy advocates warn. The Electronic Frontier Foundation says it has cracked the tracking codes embedded in Xerox Corp. s DocuColor color laser printers. Such codes are just one way that manufacturers employ technology to help governments fight currency counterfeiting.

"Underground democracy movements ... will always need the anonymity of simple paper documents, but this technology makes it easier for governments to find dissenters," said Lee Tien, EFF senior staff attorney. "Even worse, it shows how the government and private industry make backroom deals to weaken our privacy by compromising everyday equipment like printers."

Here is the Wikipedia page about printer steganography.