Sense based Intellego Magic and finding things

In your saga, when a magus casts a sense based magic, do you force them to make a Perception + Awareness roll to locate and track down what they are trying to detect? Or do you assume that if they walk within a certain distance of it, they automatically become aware of it?

For example:

Hearing the Voice of Vis (InVi 10)
[Base 1 (Detect the Presence of Vis), +0 Personal, +2 Sun, +3 Hearing)

How would you see this functioning?

For additional context, assume the magus is spending three days walking through a square mile of light forest with this spell active. Would you assume that he automatically locates any vis present? Or would you have him make a Perception + Awareness (or some other roll) to find any vis present? Does the vis only call to the magus at a standard voice range (15 paces), or at a further distance?

Your thoughts are appreciated.

Personally if he covered every square inch (or at least passed within voice range of every square inch) I would require a very easy Per + Aware, perhaps 3+, but then again we like rolling dice alot in our saga so that people feel their skill points spent are valuable. Our magi would probally cast this spell on a trusted companion that was "good at such things". If someone was even more persistent than this (walking slowly, concentrating intently for a few seconds every 15 paces or so) I would happily forgo the roll.

I've never seen any rules on varying how percise Intelligo Magic is, that is how "loud" are various things you're looking for or how "bright" if using Sight. However, I would rule that Vis is very energetic and "loud", and might be heard from farther than standard Voice range, with a higher Per+Aware roll.

Some might argue that that's why it's so very easy to locate Vis already (level 1). Good point, I would say. Roll your dice.

I'd say having raw vis audible at Voice range without a roll, barring a noisy surrounding, and further out with a Perception+Awareness roll. Perhaps EF 6+3 per Voice-range increment, no stress, no rerolls.

If someone canvassed an area, assuming he did this thoroughly I'll generally let him find any non-moving sources (probably inventing some up if I think they should be there - probably taking time off to invent them, too). Generally speaking, I'd like to think that if such a situation would come up I will already have a pretty good idea what vis sources there are in that region. I'll let him discover them. Accessing them might be a different issue altogether, as it probably requires a story.

Animal sources are more elusive, I'll let him throw a representetive roll for the whole season and decide based on that. Indeed, I would be willing to allow this as a source by itself - "You can hunt vis-bearing animals in the Enchanted Wood one per year; this will result in (die+Awareness+Perception)/3 (max 5) pawns of Animal raw vis. You may hunt more frequently and extensively to gain more, but this will diminish the future gains and perhaps the magical aura itself."

For Magical Senses , page 114 :