@Deep Waters:
I think you're overthinking things a bit. Eko will know we are a mage as soon as we cast the spell to destroy the curse, if he hasn't already figured it out. If we want to continue to pose as a Life mage (and I think we should, at least for now) then being able to passively detect vitality is not something we need need to hide.
That said, getting Eko to cast an Alarm on a twig which will make a minor ward when the twig is broken would be a very sound precaution. If something bad happens, Omo or Eko can break the twig and make us invisible to the curse spirit.
Technically, we passively detect Necromantic Energy, not Vitality. Furthermore, I'm not really worried about just being outed as someone who can magic (though there's really no logical reason for us to hide it, not when we're so adamant about helping with the curse)-- I'm worried about displaying abilities that could only belong to a Necromancer before we're ready (if we're ever ready). As I outlined in my first post, I'm very much for revealing ourselves as a Necromancer (not just as a mage) at any point once we enter the Forest proper, whether before we defeat the curse or after, since it's pretty much required for us to convince him to join "the party"-- and he'd be so incredibly useful as a companion. (I actually wanted Elana to join too
because I ship it, but I didn't even bother to suggest it because there wasn't really much of a chance we could've convinced her to come with us at that point in time-- not with the way she talked about Necromancy. Maybe if we meet up with her again after we make a name for ourselves as "good people," so to speak...) My concern is letting Eko come to his own conclusions before we can explain ourselves.
That said, you may be right about being able to pass off our ability to see Necromantic Energy as the ability to see Vitality. It allows us to cast a few spells without incurring suspicion while only being a little more risky, so there's no more reason to forgo it than there is for my suggestion. Matter of taste, I suppose-- I'd rather not directly lie to Eko, not when he's so obviously being presented as a potential companion (to put it meta-wise). Suggesting that he serve as a kind of lookout with an Alarm-triggered ward doesn't require lying, unlike passing ourselves off as a Life mage.
As for the method of Alarm-warding, if we can successfully pull off the Life mage deception, it doesn't really matter which one we choose, so I'm all for it either way. Though, if the stick approach is viable, it might be considered the Order equivalent of our Necromantic experimentation and give him a little xp, heh. Also, the more I think about it, the more I'm realizing how useful a Minor Abjuration Alarm grenade would be... I'm almost tempted to call it OP.
((I've been playing the Socratic equivalent of Phoenix Wright: Ace Attorney the entire time I've been writing these things; I think it's starting to show))