from indestructbles.com: Imagine that every time you print a document, it automatically includes a secret code that could be used to identify the printer -- and, potentially, the person who used it. Sounds like something from a spy movie, right?
Unfortunately, the scenario isn't fictional.
Most color laser printers and color copiers are designed to print
invisible tracking codes across every single printed page of their
output. These codes reveal which machine produced a document and, in
some cases, when the document was printed or copied.
The video below describes three different ways to see the tracking dots your printer produces:
with a blue light, with a microscope, or with a scanner. If you don't
have the necessary equipment for a particular step, go on to the next