Blue Rose, I am going to speak for what I think you are saying. Please correct me if I am wrong. And I hope I am not overstepping any bounds here.
There is something to notice, if you can Assense. There is no reason mundanes "should" be able to notice a difference.
He isn't saying a mundane can tell you have magic per se. See below.
You hulk somebody out with health spells and I may not know if they're magicked or on kami, but there should be a tell on that one that something's happened to their body.
Again, why, beyond your personal preferences? which aren't supported by the text I might add
Let's say a person has an Agility of 4, increased by magic that is being sustained to 4 (

. 8 is actually a physically impossible level of Agility for a human being to achieve in the world as we know it (AFAIK). When you watch this person moving, what do you see? This level of grace and coordination is, simply, unnatural. In the world of Shadowrun, then, where this kind of thing can and does happen, it will look like this person is wired, magicked, possessed, maaaaybe certain kinds of metahuman, maybe a vat baby, etc.
What Blue Rose is saying, is that the magic itself is not visible, but the change that it has caused is subtly apparent. He is not saying make detection of magic two rolls all the time so much as trying to communicate that the initial casting roll isn't the only time when the spell can be recognized as a spell, even without Assensing. The cue is NOT definitive, however. You examine this person's unnatural grace, and you have to come to your own conclusion for WHICH OF THESE THINGS IT IS. Natch, omae?
He is also not saying there is a second roll to spot the illusion in the case of a Mask - just the opposite. He is saying it is
even less likely you get a second roll to resist, because the high Force of the spell makes you more "solidly covered/enclosed" in the effect of the spell. Someone "sees" the magic as MORE real and thus harder to differentiate from reality precisely because it is a higher force.
It is not a stretch, then, for something like Increase Strength (when cast at higher levels) to visibly increase muscle mass that is evident externally. It is a Health spell, after all. If Heal can stitch up and seal wounds, then Health spells do have some slight crossover into Manipulation when it comes to body manipulation. But Shapechange is Manipulation. So where is the line? The book doesn't quite say. I'm inclined to think that the muscles would increase in apparent size a
little bit when increasing Strength by 3 or 4, but how is someone to know that isn't your natural muscle size if they didn't see the spellcasting? Stretched clothes might give an idea, but I wouldn't say that it stretches enough to, say, ruin Custom Fit, so the change wouldn't be that noticeable. Maybe if someone already knew you very well, or saw you earlier that day. In the end, rules can't cover things like this near as well as individual tables can in my opinion. ^^
Your examples in bullet point two seem to me to be illustrating the point I think he was also trying to make. Like you guys are trying to say the same end result but from two different perspectives, maybe?

This isn't the sort of thing that rules are really meant to set down, aside from the obvious "Armor makes you glow" kinda thing (which, incidentally, is probably because it's an entirely different kind of spell than Combat Sense or Physical Mask, and there are different assumptions about how those spell types work. Or even within the same spell type - it's not like Control Thoughts or Influence logically have visual components to them, and if they did, they'd suck for some of the same reasons Armor sucks, i.e., the obvious "geek the mage" sign in neon they would create).
A fair point indeed.
Frankly, the difference between Automatics 1 and Automatics 6, or the application of Improved Ability, or whether or not someone has vision mods in either ware or contacts, has an effect on the world and how such characters would be expected to act in the context of using those things, but not one that's obvious if someone puts no effort into descriptors (and not everything needs such a descriptor constantly, I'll add); otherwise, it "just" affects numbers, which in the end DOES affect the setting as it effects results, turning otherwise-ordinary people into superhumans with the details and effects they can achieve.
Not sure why this is controversial let alone a big deal; that's not exactly an uncommon thing in RPGs, that effects seem bland if the player doesn't try to make them appear interesting IC.
Eeeeeh....but not everyone is good at that sort of thing. It is and it isn't necessary to include descriptors. I'd say it is for the best to have a rulebook section ON description and how to adjudicate what the numbers MEAN (aside from the Skill chapter table about 1-12 ratings, etc), but I don't really need more than that. Some might, though.
What I am unsure of is if Blue Rose is actually trying to advocate a change in how the book should be represented, or rather that he is trying to explicate his subjective viewpoint of how magic works/should work in this setting in a clear way. I'm not seeing his end game just yet.

Also, I particularly agree with what you said above that I put in
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