Now that you mention it Sever, it does sound familiar. I thought it occurred outside the Bonebiter quest as well though. Maybe its only the undead that notice you, but I thought that other creatures did too - something about casting invisibility in front of them that negates the spell I thought. Perhaps not. Trouble is, when it happens, you're normally too busy trying to stay alive to notice what might have got you killed in the first place.
