kris
Arcane
I've never ever had this happen in about 30 missions. I'd be glad if I was proven wrong but it really seems like the aliens are just static groups that appear when you move into their LOS.
playing on Impossible.
They group is indeed just a simple marker until spotted. You will never get fired at or ambushed before spotting the enemy, you can loose sight of it momentarily though, since they get free move.
They move, but they never spot you and shoot at you in the same turn. I've had many occasions where I end the turn having not seen any aliens. Some patrol marker paths into the range where it turns into aliens, they move into view. The patrol gets their "we see you" cutscene and another free move. What is happening if I end turn not having seen aliens, then aliens move into my range during their turn, if that isn't a patrol?
Rotating spawning points. I am not saying it is that, but I think it is very likely going by my experience of the game.