In combat, creatures are aware of everything obvious around them and are assumed to be constantly looking for threats in all directions. There is no Facing in D&D - this is why rogues don't get "backstab."
While in turn based mode, enemies should have detection radii - a smaller one for hearing and a larger one for sight. This prevents cheesing the system by moving all the way past the enemy before they even get their turn and are allowed look around, thus allowing a Fighter with a Dex of 8 in Heavy Armor to sneak past for free.
While in real-time, enemies should have a vision cone and a smaller (~10-15 ft maybe) hearing radii. Players who want to play the "sneak past enemy's vision cone" game can thus do so in real time, where it's an actual challenge instead of the triviality it is in TB.
