I’ve had time to play a few missions random the random map and this has cropped up in each. I’m unsure if it’s a bug or not, hence bringing this up under feedback. It’s been most noticeable on the boss when it pops up but the smaller bugs seem to do it sometimes, too.
The boss just seems to stand still where she spawns until it gains a line of sight to at least one of your soldiers, after which it seems to act normally.
I’ve encountered a similar problem with the crabmen where, after you get the enemy spotted message, if you duck out of LOS, and then reacquire later, the alien will be in the exact same place - even down to facing in the same direction, even if you wait many turns.
It also seems that if you engage some of the smaller enemies at extreme range they won’t react to incoming fire, even if they take damage.
As far as my memory serves, all this has occurred when my squad has not has LOS to any other enemy.
Do the aliens do anything when they are out of sight, and if not, are is that something that has yet to be implemented or do they wait to be stumbled upon?