The United States Secret Service is a federal law enforcement agency of the United states in the United States Department of Homeland Security. It is divided into two groups which are the Special Agents and the Uniformed Division. To look for an old friend, simply contact the secret service through their website http://www.secretservice.gov/contact.shtml.
Depends if he is on a current assignment attached to a security detail or not. The secret service does lots of stuff like solve crimes that have to do with money. If he is on that kind of assignment, all you would need is the state he lives in an a phone book to call the office.