I hate to give such a simple answer with all the conspiracy stuff about Fredo but after watching the movie twice in the last two days this question came over me so I checked. It appears that when the black dressed character (which by the way would be kind of a tipoff that he might do something deadly since he looks like death) comes into the room there's no one in particular watching, but there are people in the next room.

As he's about to smother Roth, one of the nurse types seems to be leading the military people into the room. In other words, she spotted him. He thought no one was looking, someone was. Why were the military types there in the first place? I think there were nearby to guard Roth and Micheals assassin thought he had an opportunity with the party in the next room.