I have ran past them with sneak/invis up and gotten aggroed right away. I don't quite get it, but I swear I've also run past with sneak/invis/deodorize and still grabbed aggro.
Scent is the way they track. Which means if you like sleep it and go far away, it will find you by pursing your scent. It is also how they blood aggro if I am not mistaken, which I think its also prevented with Deodorize. I havent tried myself, have to test that theory.