And if you see one in a map and don't manage to kill it (or do), they won't reappear if you replay the map. Like asw said, they seem to be strictly random.
My experience of meeting this. At G13 during grinding "battle won" i send all waves instantly(8). Every time i got black flickering, 15 times one by one. Later i set +5 waves, send 10 waves, it's appeared again. I killed it and from this time it's never respawned at G13, so i am sure that it can be killed only once per field. Now i killed them 25 times and they start to appear a lot later than usually. One time it's appeared along with shadow(at 20 wave), once saw at 60 wave.