The Electronic Frontier Foundation started a project some time ago to decode the hidden tracking information that certain colour laser printers embed in documents. The results are out and EFF's findings are rather interesting. The info is hidden in small yellow dots that are too pale for the human eye to readily discern, but are visible under magnification and blue light. EFF reports here: EFF: DocuColor Tracking Dot Decoding Guide.
Thanks to Boing Boing for the link.