Theres your problem... The trigger activates when you are in it and are detected from anywhere on the map. Thats why the mission plan says its better to alsow sneak into the arfield. Maybe I will set the trigger to be deleted after task is complete in the future... maybe...
Added 13 minutes later:
Ive released small update Alpha version 0.2 for this mission.
-Replaced init.sqs with init.sqf (fixed the Infotext problem).
-Paratroopers now have to be called by radio when the AA systems are destroyed.
-some oather changes.