Just realized something interesting, after the umpteenth rewatch of NoTD. If one skips to about time-stop 17:10 into the episode, you can view the scene where the door to the Doctor’s tomb opens, and River walks out, and announces that she said the doctor’s name (To noone in particular). Well, as she moves off-screen and the scene cuts, you can watch Mr. G Intelligence’s eyes follow her, as if he knows she is there.
It is well established at the end of this episode, and inferred throughout, that the Doctor can see River as well. Is she not quite as invisible as she thinks? Especially since GI’s “Whisper Men” were in the conference call, can he see her as well? It is just a telepathic link, but everyone present in the episode save the Doctor was there. Can they all see her?