If they knew where to look, which they do not. However, as they progress perhaps the source will become... more obvious.

Wrathion digs in his pocket for one of Karsa's wards, steps to the side and affixes it to the wall of the hallway. One hand lifts to channel magic into it, strengthening its hold. Narrowing it down, in some manner, because perhaps this will not banish the entire presence here but it might lift the immediate haze in this room and allow them to select a new direction.