I knew about this.

Did you know your printer probably prints a hidden code which identifies it *specifically* to *your* printer?

There were concerns that the new color printers were good enough to print decent counterfeit cash. To counter that, each printer is made to print an ID mark, specific to the printer (it would be something as simple as 3 red dots in a specific location, such as upper left corner). Not all printers did this, and I don't know if they are still doing it or not, but... just another one of those things.