B757 flight attendant discovers enroute that two Federal Air Marshals are on board that have not been identified to the flight crew.

Narrative

Agent closing out -- was in a hurry -- kept trying to close aircraft door before required Flight Attendant duties were complete (Examples: overhead bins closed; all luggage stowed properly; and exit row briefing complete). After takeoff (cruise altitude); I observed customers exchanging minimal conversation. I rechecked manifest/paperwork. Nothing was written (informing me of Federal Air Marshals on board.) The passenger mentioned confirmed they were Federal Air Marshals. I notified the Captain and the First Officer of this situation. They were unaware of any Federal Air Marshals on board. Agent did not inform anyone. I notified the other crew members of the Federal Air Marshals presence and location on the aircraft. This agent was negligent in omitting this important safety information. Correct and required procedure: 1) The agent informs Captain of Federal Air Marshals on board. 2) Agent writes a notation at the top of my departure manifest with Federal Air Marshal information and seat numbers. This omission compromises crew communication; crew coordination; but most importantly compromises the safety of flight.