Changing jobs is something unknown in your universe?
Jasper
This has been possible with laser printers for a long, long time (the drum, especially when not brandnew, leaves a distinct vertical microscopic pattern on the sheets), and to a certain extent, there *are* things that can be proved about inkjets -- a printhead/cartridge combo leaves horizontal patterns, and some of the paper advance mechanism can leave a certain pattern vertically, or at least tell the researchers how well used a certain printer is. This is basically a variant of the old trick found in many detective novels of seeing that a certain letter on the typewriter used to write the suicide note/ransom letter is worn, and then finding the Underwood with the corresponding damage.
There's also tech in both newish printers and in newish Photoshop and such (and possibly even scanners) that actually refuses to work on counterfeit bills (or passports, for that matter). The banknote-detection-algorythm is fairly subtle nowadays, but google on 'eurion' for an early and deprecated example of how that'd work.
The plans (already implemented?) to put a chip in newer printers that actually deliberately encodes the serial number of the printer in all output, to aid law enforcement in forging cases (and also, say, ransom notes and such), however.. those are getting creepy.
Jasper
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