The “When touched”-event of a NPC fires only, if the hero touches the NPC or the NPC touches the hero from the front. If the NPC touches the hero’s back, it will not fire. Therefore, we cannot e. g. let a guard catch the hero as an intruder – at least not, if the hero never watches into the face of the guard.
If you want to add this, be aware that existing games may rely on this behavior. Therefore, we should have a switch for it.