I don't know what causes it, but when a skeleton sees you through a door, window, trapdoor and walk away while it still sees you, you take damage if you're 6-12 blocks away, not just damage but constant damage, and they can do it too when you're behind a wall but where the skeletons are 2 block above as shown in the screenshot. It is confirmed with a skeleton, but i don't know with the other hostile mobs