You could encode information as slight "mistakes" in the print. An extra dot here, a missing dot there, some slight "printing error" in some place.

There could be ways, but as you said, it's to combat money counterfeiting. Like the Euro constellation detection in scanners, PhotoShop, and printer drivers. (fun fact: it's quite easy to bypass, not sure why they even bothered... it only stops people who put 0 thought into counterfeiting anyway)