Remember that Mostin, Ortwin and the whole group were Mind Blanked through the entire scene. Sep actually mentions that Irknaan could scry their location, but not them. So all Irknaan knows is that *someone* in the group turned his retainers into dust, paste, statues, flounders and gibbering idiots. He has no idea it was Mostin.
Mind you, its quite possible that he *realised* they were Mind Blanked, and dispatched a single Sidhe on a griffon with Eyes of the Eagle to monitor the whole thing from a distance, but this seems a bit, well, prosaic for an umbral demonic Faerie, don't you think?