B777 Flight Attendant reports discovering during boarding that FAMs may be aboard but unannounced to the crew.

Narrative

Someone brought a Law Enforcement Officer on board identifying him to me and cockpit crew. Boarding Flight Attendant overheard her say there were 2 more armed passengers on board and asked me about it. I went to the Captain to ask him; he said no. When the Agent came to the cockpit to get the OK to close the door; the Captain asked her; 'Do we have FAMs on board?' She said no; and he told me there were none on board. After takeoff; the man in XD told the #6 Flight Attendant that he had overheard her talking about the FAM incident the week before; and was she on the flight? She said yes; and he said he was a FAM and that his colleague was on that flight. She was shocked. He then told her he was a working FAM and so was the man in YD. She told me; I contacted the Captain. He contacted dispatch; and after about 30 minutes they confirmed the presence of 2 FAMs. In light of what happened the prior week with the fake FAM; this failure to ID the FAMs to the flight crew is an extreme failure on the part of the Passenger Service Department and should be addressed. If we had not found out the FAMs were there; and something had occurred; the situation would have been chaos.