Hmm ... not present triggers can be a pain. However, the problem with naming the loons and doing not alive is that if one of the loons flees he could be in a wood half a mile away. You will never, ever find him.
Much better is to have a not present trigger drawn over a carefully chosen area. (Sometimes you need more than one to get the right shape.) The last few defending loons' behaviour should be controlled by the mission designer: maybe they get ordered to flee, or joined into one group which is not allowed to flee, or whatever. There are lots of things you can do.
The real trouble with not present triggers is that many mission designers are not sophisticated enough to use them correctly. Often they are far too large for a start. They should always be part of an "objective complete package" which includes waypoints and other triggers.
In this case, if the radio station is small, then a small not present trigger is probably fine: the last loons will flee only if they have detected you so whether you kill them or not is irrelevant. And if the area is small, finding the last enemy should not be a problem.