The Shamanic Mask is an optional way to have your character's spellcasting be noticeable. For your character, you can have their method of Spellcasting be gestures, spoken word, shamanic mask, some other sort of "visual manifestation," or any combination of the previous.

That actually seems pretty clear cut. I know I will be requiring mages in my own game to make some kind of visible 'gesture'.

The magical effect may not be visible, a manabolt thus being invisible upon casting and until impact with the target, but the gestures required to cast them are still physical.

The funny thing is that a lot of that is just fluff, not game mechanic. Magic is like any other skill; you'll always perform it how you learn it. If people learn division by writing it out, then they'll naturally write it out when they need to do division. If you learn magic by concentrating and asking the spirits in Creole to lend you their strength, then TA-DA, that's what you'll do. But that's not the necessary part of magic; only the concentration and molding of mana is important, much like in division where the only thing necessary is the mathematical operations to acheive the result.

In my book, people notice magic due to the magician's intense concentration; perhaps they chant under their breath, or their eyes roll back slightly; whatever. When the spells cross the magical FORCE 6 threshold, well... then the air around the mage starts rippling, and you feel the mana being channeled and shaped around them.

Not hard, even for Bob the Pedestrian to spot. And while Force 5 is very beefy, its hardly what I'd call immensely powerful.

Not exactly - we will assume Bob is on the higher end of average, with a 3 Intuition. No Perception skill - he's just some guy.

So, we have 3 - 1 (defaulting) -2 (not actively observing) = 0. Unless he has vision enhancements (which are cheap, but 'that guy' is unlikely to have them), he will not notice spellcasting without GM fiat; aka -14 Threshold (Force 20) should be noticeable as long as you have some form of sensory abilities & are on the same block.

or none of the above because unless you have gease or centering hand motions, speech or some other sign are actually not required.

But spellcasting is noticeable (as has been quoted in the rules), so the Magician must do something in order to make Spellcasting noticeable.

If you don't want your mage to gesture, hand wave, or manifest the shamanic mask, then their magic creates a more noticeable "physical" effect on the world: wind rushing, visual distortions, distinctive smell, etc. The book lays out the crunchy rules for noticing spellcasting, there is no option to make your mage's magic "unnoticeable."

Basically what it comes down to is:

Magicians are not required to perform any action to cast a spell, unless they have the Geas quality.

Most magicians will perform some kind of action while casting anyways, as a focusing technique.

Regardless of if you perform said actions or not, spellcasting is noticeable - I have always gone with the explanation of observers noticing the mana gathering around my character (remember, it is a Perception test, not a Visual Perception test), with no visual or audio effects, although observers may "see" it, lacking the ability to understand what it really is.

By RAW D&D rules a fighter with a rank in Perception and a +1 from stat rolls his d20 versus a target number of 15 + spell level (he needs a 13 to notice a 0th level spell).

Basically what it comes down to is:

Magicians are not required to perform any action to cast a spell, unless they have the Geas quality.

Most magicians will perform some kind of action while casting anyways, as a focusing technique.

Regardless of if you perform said actions or not, spellcasting is noticeable - I have always gone with the explanation of observers noticing the mana gathering around my character (remember, it is a Perception test, not a Visual Perception test), with no visual or audio effects, although observers may "see" it, lacking the ability to understand what it really is.