1st that they could do is if literally you can see the NPC, the NPC should also be able to see you from a distance, pursue and engage you.
I have to give you credit, this is one of the most insightful posts I have seen in this category. #1 might be too hard to code, but it a good thought. #2 should be a no-brainer.2nd. When the guards engage an enemy it should show in the map like what normally gets shown when there is a fight anywhere in Cyrodiil.