Stenographic anti-counterfeiting ("secret") marks

Many modern color laser printers mark printouts by a nearly invisible
dot raster, for the purpose of identification. The dots are yellow and
about 0.1 mm in size, with a raster of about 1 mm. This is purportedly
the result of a deal between the US government and printer
manufacturers to help track counterfeiters. [1]

The dots encode data such as printing date, time, and printer serial
number in binary-coded decimal on every sheet of paper printed, which
allows pieces of paper to be traced by the manufacturer to identify
the place of purchase, and sometimes the buyer. Some are concerned
that this is a threat to the privacy and anonymity of those who
print.[attribution needed]

 http://en.wikipedia.org/wiki/Laser_printer#Color_laser_printers