I suspect the trigger would have to be "triggered" by unitA. So, for example, if unitA was East then the trigger Activation box would have to be East present or East not present. You might have to set the trigger to repeatedly, otherwise it might be triggered by somebody else before unitA arrives.
If unitA is a group leader then group the trigger to him, as if it was another loon, and you'll get new options in the On Activation box.