Actually, the "detected by east/west" triggers will go off once someone on that side has fired a loud enough weapon, or fired a weapon enough times. Not sure if this directly relates to when the AI will pull their weapon off their shoulder though.
Another way is to check if the AI group is in "combat" mode. AI groups will always automatically switch to combat mode when they detect the enemy, unless they are in "careless" or "stealth" mode.