Although I admit that coming from DnD games, detecting with mechanics seems to make no sense, after thinking about it a bit, it makes sense to me.
I think that, to detect a hidden trap, or a hidden mechanism, you have to be familiar with hidden traps and mechanisms, with how they are constructed and installed; from that point of view, it makes sense that it is the mechanics skill that allows you to realize there's a trap in front of you, because if your mechanics skill is high enough, you know what to look for. It would not make sense if it was stealth, because the skill to hide yourself doesn't suddenly gives you trap engineering knowledge.
That said, I agree that conflating the "searching for traps" and "hide in shadows" stances in the scouting mode has made things confusing, one tend to intuitively assume your character is just trying to hide, not trying to detect anything, and then think "why do I have to hide to pay attention to traps?". But you're actually scouting, which for me could be translated to "just being careful and attentive, watching your steps in case there's danger".
EDIT: oh, the conversation was mostly about non-trap hidden items. Yeah, in that regard it doesn't make sense. My bad, I read too fast.
EDIT 2: although it depends of the way those non-traps are hidden. Like, something hidden just behind a curtain shouldn't need mechanics, but something like a hidden room behind a fake wall that doesn't make architectonical sense with the rest of the room could be something a mechanics (think engineering) knowledge would help finding.
Edited by pedroantonio, 27 March 2015 - 01:40 PM.