Again the whole power lv for cost of item is ridiculous argument. Not all spells or items are balanced, most specifically not around each other. Again deathwatch is a supernatural ability or a spell....why the flying f**** would it work just by going yup he has blood on his clothes and it looks to be his? Clearly it functions by reading the life force in individuals which would be perceived as an aura, guess what this aura would appear even if you are invisible. It's like arguing pcs can't see an invisible monster underwater...he displaces water and creates air bubbles.
Seriously. .. making deathwatch stupid and particularly useless (again most parties tend to kill things in combat or use heal checks after)