Well, in OFP i've been using the following technique when trying to
achieve what you're after here;
A trigger detected by West
Another trigger detected by West (same location as the first one just a little
bit bigger)
Then you have two triggers which should return the same units inside
their triggerlist (in case all living West units are inside the inner trigger).
Now once the a unit is in the bigger trigger's list but not in the inner
trigger's list, it has left the area.
~S~ CD