I mean, I don't know if you'd call it a watermark in the sense of the way it is in money. Just identifying marks. However, printers leave a a signature fingerprint similar to the way you can trace a bullets patterns back to the gun that fired it.

I don't know how much work it would be, or if it's even feasible, but ballots from a specific printer CAN be traced back to that printer.

Like if you printed out a ransome note or something from your printer and the feds suspected you. They could print something from your printer and then match up marking that you can't see with the naked eye. They obviously used a much larger industrial printer, but I'm sure it leaves some type of fingerprint as well.