All colour laser-printed documents can be traced
According to this article, new colour laser printers output a non-visible (to us) serial number and date-stamp. At least Xerox shares customer lists (and hence serial numbers of the printers they own) with the Secret Service if asked.
``Robert Lee, a computer science student, spent the summer after graduating from Yale researching these yellow printer dots, trying to figure out which companies were using them and what they might mean. My coworkers at the Electronic Frontier Foundation (EFF) helped him along, giving legal and technical pointers along the way. The result of two months spent peering at color laser copies with a blue LED was the discovery that only one model in his study doesn't use the yellow dots: the Xerox TekTronix Phaser 7700. But most other Xerox models do, as do all the ones he tested by Canon and Toshiba.''
[..]
``For example, how do we know they're only using these printer marks to track counterfeiters? What if they're also tracking people who copy what they think are anonymous political flyers or pamphlets? Or people who post flyers announcing meetings of Muslim groups?''