I use simple idea in my missions (none released yet, needs polishing, dont have time). A trigger over east (enemy) territory (or the whole island, whatever).
Name: detectedguys.
Repeatedly activated.
West detected by east.
Condition: this.
On activation: defenders move position (list detectedguys select 0)
On deactivation: defenders move position object xxxxx
defenders is the group, which is on stand-by for enemy (player)
xxxxx is the number of the closest object of defenders waiting position, could also any position be used, just easier this way.
If you are making a single player mission, then in some occasions you could use player in stead of (list detectedguys select 0) in the trigger.