"When you print on a color laser printer, it's likely that you are…

"When you print on a color laser printer, it's likely that you are also printing a pattern of invisible yellow dots. These marks exist to allow the printer companies and governments to track and identify you -- presumably as a way to combat money counterfeiting. When one person asked his printer manufacturer about turning off the tracking dots, Secret Service agents showed up at his door several days later."

http://seeingyellow.com/