If LHO was in the sniper’s nest (I believe that he was, others can believe whatever they want to believe), I think he would have been aware of the elevator noise when Norman and Jarman took it to the fifth floor. I believe that LHO would have been able to hear Norman and Jarman when they stationed themselves on the fifth floor, just below the sniper’s nest. If Norman and Jarman said anything (they probably did) I believe that LHO could have recognized their voices and known who it was just below. LHO would know that in order to access the elevator on the first floor Norman and Jarman would have been within sight of the door to the domino room. In my opinion, LHO didn’t need to see the two of them outside (nor did he need to be in the domino room at the time) in order to know these things.