This is definately true as I have seen it first-hand. The Xerox color printers lay down an imperceptible sequence of micro dots in yellow toner over the entire page which can then be traced back to that specific printer. I was shown a copy of some Malaysian counterfeit money which was sent to Xerox by their government to obtain the serial number of the machine it was printed on. After blowing up the image and altering the contrast, the dots will become visible. If I'm not mistaken, they can also be seen if you keep making a copy of a copy of a copy.