Sometimes when AI are hold up in buildings in combat they tend to go prone and look though the walls and seem to go into tunnel vision. its common to walk in to a building a find all the AI prone looking at a wall and not notice you breaching.

It seems there is a issue with AI vision where they don't detect the wall right in front of them and realise there is no point in looking that direction. AI should rarely ever be in prone inside a building and keep eyes on windows, doors and stairs