I do know the scene pretty well and also that Graendal did it, I'm just asking about the circumstances. Point being that there were so many capable channellers around that channelling within the palace must have been noticed, especially if it was strong channelling for balefire - but it seems...
I know, that Graendal claimed to have killed Asmodean. Do we ever get to know where (not in the room he searched for wine for sure) and how (balefire?)?