Board Thread:Warhammer 40k General Discussion/@comment-174.29.74.18-20130615110043/@comment-3580035-20130620082913

Deathwatch: Mark of the Xenos gives a historical account of how a Tau Commander instantly knew when the last Ethereal on the planet died, and also instantly knew as soon as another Ethereal arrived without any form of communication/sight; he just knew.

Pheromones simply don't work, as they can't go through airtight armour/Battlesuits, basically making the theory pointless as it would mean Ethereals only have an effect when helmets are removed. Whilst they do emit a form of pheromone to enhance control, it is definitely not the main cause. The original theory was from simply the speculation of a Imperial scholar.

Source for ethereals warning the Tau not to delve into it? Never seen that before.

The point of ethereal control is that it is mysterious. No one knows what gives them such power, but have it they do; and its not psychic in nature. The Imperium would've noticed that instantly if it was.