In the On Activation field of the Transport Unload waypoint, put the following code:
AtCamp = true
In your init.sqs script (create one if you don't have one), put the following code:
AtCamp = false
In your trigger's Condition field, put the following code:
this and AtCamp
Explanation: As long as the AtCamp boolean is false, the trigger's condition will not be met even though you are not present in the trigger's area. Once the helicopter's Transport Unload waypoint is activated, the boolean will become true, which will then allow the trigger to check if you are not present in its area. This will only work if the waypoint drops you off within the trigger's area, though. Otherwise, the trigger will activate the moment the waypoint is activated, which is not what you want.