So if you're stealthed and they're stealthed do you not see each other?
Well, with this imaginary system I have in my head, no enemies will ever be "truly" invisible by simply using stealth. In fact, enemies may not even use stealth unless certain factors are already met; such as: lighting, whether or not the player is in combat, how many other NPCs are around, etc. I wouldn't want it so enemies aren't visible at all unless they're using illusion magic. Think of it more as they're using the environment to sneak up on you in much the same way you would sneak up on someone. Things like, hiding behind a boulder, or right around the corner through a door in a dark room. If you turn and look straight at them, then you'll probably be able to see them and you'll "detect" them.