Where -- and how many -- units the AI keep in their territory is one of my great puzzles with them. I see an AI sponsor getting thrashed by another AI, yet it keeps a squad of ships parked near polar ice or where an alien next *used to be*. Parked. Not coming home to help, just parked. They shuffle their land units around, without any pattern I can see.
In other franchise games, I always escort my settlers. I've been known to send BERT colonists unescorted, if I have a unit or two in the general area and the site is within a few hexes of my borders.