Currently, the "Warning!" rule allows a model to change its facing towards an enemy model within its Zone of Control, but it does not grant an ARO...
The situation in question is the following: an enemy trooper is standing with his back to a window. My trooper shoots the window with a Missile...
In order to use "Warning," the user has to have not been activated by ARO and either needs to be the target of an attack or within ZoC of someone...