To me for most enemies it would seem very illogical if they flee.
Undead, constructs and similar enemies don't know the concept of death, so the won't flee.
Gith, all those religious fanatics and those sworn to defend something with their life will fight to death.
Creatures attacked in their lair generally don't flee either.
People who are in the only location that is somewhat save have no other choice than to fight.
The only enemies that come to my mind where flight would be a reasonable option are Lorroakan, the Zhentarim and some minor encounters.
