There are supernatural "perception" abilties like magic sensitivity. I've been wondering if they are always on or activated and deactivated.
Does a guy with magic sensitivity walking past a vis source or a magus with an active parma or THE ONE RING realize that he just passed sth magical?
a) Is the always on or not thing the same for all these virtues?
b) If not, how exactly does Magic sensitivity work?
Always on, like sense holy/unholy, second sight, premonitions and similar effects. Perception stuff is always on, generally. Same for awareness
Cheers,
Xavi
In my games it's an active ability, so you have to announce you're using it.
I would think that it's an "always on" ability, the same way I would do Awareness, Sense Holy/Unholy, Second Sight, or any other type of innate sensory ability.
I'll come down on the "must be activated" side of the fence. In my saga use of these extra-perceptions requires a moment of concentration to "switch focus" to these alternate means of perception. Some people may choose to keep their focus on mystical rather then mudane perceptions, but such people are usually at least a little insane.
In a world overflowing with invisible spirits (guardian angels and tempting demons sitting on every shoulder, nature spirits in the trees and streams, faeries flitting about on their own strange errands) sensing them constantly is a short path to madness or at least behavior odd enough to be seen as madness by everyone else.
I do sometimes allow "glimpses" of things, mostly plot relevant things, even when characters aren't actively using their abilities... most as a way to remind forgetful players of exactly what they spent that Virtue and all those experience points on. 
In our saga they are "always on" like normal senses. However, like normal senses, you can try to focus on them (improving your chances to sense stuff) or instead try not to pay them too much attention to avoid getting distracted.