Well, the player with high sneak skills can sneak with full plate armor, why can't the enemy?
I think that's rather dumb, too. You shouldn't be able to sneak (and steal/pickpocket) effectively with heavy armor no matter what your sneak or heavy armor skills are. Effective sneaking should need plain clothes, or light armor with some training.
Orges and bears also can sneak attack if they want to, they also have tactical brain.
Not when they're running at you full-tilt.
So the enemy always don't know the hero sneaking on them, but the hero will always know there is enemy nearby because of their music
Enemies can spot you before you reach them if you're not sneaking or invisible, even from behind (and even if you are sneaking, you need a sufficient sneak skill).