Some mobs have a perimeter that they stay in, others will chase you through an entire dungeon to kill you once you aggro. The ones that have a patrol area return to that zone once you get far enough away, the others do not. I had a Healer in a bad Aurum Vale run a few nights ago that chased some Diremites all the way from the final Morbol pit before the boss to the main room that you first enter upon starting the dungeon. After the Diremites caught up to her and killed her, they proceeded to go after other people.