This has been an issue for a long while but since it ended one of my good runs today, I figured I'd post whine about it because it's probably 'unfair'
Ever start in an arena-type level and get surrounded by Barons or Revs and use a phase device to escape to a side of the level...just to find that those same enemies come charging at you a few turns later from the center?
It's because once an enemy sees you, they maintain their knowledge of your position AFTER you phase. Verified using Int 2 and getting surrounded...use the phase and landed in a corner...and I waited, watching those enemies in the center run directly toward me to the corner.
EDIT: This is most noticable with enemies that tend to maintian a long-term fix on your position after spotted (Barons & HKs) but I've had Revs and Former Commandos cause trouble as well.