I wonder if printers that I use does this.
The big question is: does it matter? I think it does. I think it is a breach of common decency and unethical to have such covert stuff done. I have no issue if this was done on legitimate currency notes so that we can tell, but for every document printed? Again, if this printer is in a controlled environment, I have no issue. In fact, I think the printer companies might want to provide it as a customer settable option so that the functionality can be more widely used.