Do you realize that something in your office/house might work for Secret Service? According to EFF, print outs from Xerox DocuColor have hidden information for forensic tracking. See, these yellow dots look harmful:
...BUT someone can reveal that e.g. it was printed on June 21st,
2006 2005 !
So far, only code from Xerox DocuColor printers has been cracked, though EFF researchers believe similar things appear in other printers as well.