Enemy Spawning Bug/Line of Sight

During a haven attack mission I had cleared all visible enemies and when the “Pandoran Activity” went off, it was done almost instantaneously. Assuming whatever the enemy did, it hadn’t moved. After searching for 10 minutes one of my men spotted the creature apparently through the wall (no clear LOS at all) and it was on the 3rd floor of a building. It was in a two space large area, walled in, behind a ladder. After a mix of grenades and heavy weapons fire I opened up a hole in the floor to shoot at it from outside. Then I damaged a wall behind it, it moved out of most of my units sights, except for one who could apparently see it through the undamaged wall and take it out.