Bruce Schneier pointed to a sneaky feature present in some color printers, like in Xerox DocuColor series. They print a code on every page, allowing the authorities to track when the document was printed and with which printer.
A little while ago, I ordered some keyrings with my company name engraved on them, to give out to customers. Just a PR gadget. Now it turns out that they’re probably close to perfect to check color printouts for those hidden codes. Some color printers include hardly visible light yellow dots in the printout that code for date and time, including the printer’s serial number. That expensive color laser you paid for with your hard earned cash is ratting on you. Check out this picture and you’ll see that they’re using blue LED flashlights very similar to the light built into the keyrings I got. How incredibly opportune.
To see how my keyrings look, see the logo at the top of my www.ssdes.com page. That “logo” is simply a photo of the keyring. The round insert on the left, with the black button at the center, is the blue LED flashlight. It’s strong enough to light your way on a dark night with. And, I’m sure, blue enough to detect those yellow dots with, even though I have no printouts here to test with.