A study done by Electronic Frontier Foundation finds that most color laser printers add an identifying code on every page you print. This code is actually microscopic yellow dots printed on each page in a grid pattern. Normally these dots are invisible to the naked eye and can only be seen using a blue LED light.
The information in the yellow dots varies, some have just the serial number of the printer and others also have the date printed. On the picture below you can see the date and time when page was printed and the serial number of the printer.
Originally the technology was implemented to help secret services track and find counterfeiter who use color laser printers to forge money and other valuable documents. But who know how seriously this can compromise our privacy.