Basically, all the sneak level on an NPC would do is alter the volume of his/her footsteps. When sneaking, they will not go into "battle mode (loud music)" and if they hit you it will be considered a sneak hit (oh, actually, that's the other thing their sneak skill will affect). Basically, to 'detect' them, you have to turn around so that they are within a certain radius of the centre of your view (which would be a smaller region than your default view so you don't detect them if they're in your peripheral vision) and then they will no longer get the multiplyer. They would also be designed to never really give you one hit kills (Let's just say they svck at headshots), always leaving at LEAST a tiny bit of health (though it would vary greatly how much health exactly)so that you have a chance to survive. Also, these NPCs would have to be positioned only in dungeons and quiet wilderness areas, so you can't be killed because you didn't notice one person in a busy city amongst the millions of footsteps.
What do you think?