That the assassin nicknamed Slayer would be intuitively obvious in RJ's sense, because of his profession, his name & the fact that he quickly killed other people associated to the Shadow in pretty secluded places out of thin air in the books before. IMO, one can figure all those things out, combining the clues, like RJ said.

The explanation for Graendal being intuitively obvious at this time is the same like it would be for Sammael & maybe other Forsaken, and at least Sammael would be even a bit more obvious, because he actually stated that he would kill Asmo if they'll meet. Thus, one certainly can't say that Graendal was intuitively obvious at the end of TFoH.