Not the best screenshot, this is the area after the Chaos Warrior hall and a blockade, all the way up after all the stairs. To my left is a narrow passage to the door into the next hall, somewhere behind and to my right is… A statue or something?
On more than one occasion a sneaky Blightstormer spawns and does his heresy, behind the wall/glass window I’m looking at and out of view/reach.
Disclaimer: not sure if it’s relevant to BBB, I think I encountered this on live and doubt BBB would cause this, but still.