If you set up a Trigger such as Blufor Detected by Opfor with a Timeout (or Countdown - tried them both), the trigger will activate even if all the detecting units die before the timeout completes.
So if you want to create a mission where an alarm goes off only if a enemy has time to 'call it in', it requires some less-than-elegant scripting.
{F20608}
-Place 1 Bluefor and 1 Opfor riflemen near each other.
-Create a trigger for when Opfor detects Bluefor with a 5 second timeout. Display hint when it activates.
-Play the mission and let the Opfor unit detect you (it starts to aim).
-Kill it in less than 5 seconds.
-Watch as the trigger sets off anyway.
I'll attach a repro mission with an unarmed OPFOR soldier.