And.... my ignore list grows again.
More and more I think too many want a video game and NOT a tabletop game.
per·cep·tion
/pərˈsepSH(ə)n/
noun
noun: perception; plural noun: perceptions
the ability to see, hear, or become aware of something through the senses.
"the normal limits to human perception"
•the state of being or process of becoming aware of something through the senses.
"the perception of pain"
synonyms: recognition, awareness, consciousness, appreciation, realization, knowledge, grasp, understanding, comprehension, apprehension; formalcognizance
"our perception of our own limitations"
•a way of regarding, understanding, or interpreting something; a mental impression.
"Hollywood's perception of the tastes of the American public"
synonyms: impression, idea, conception, notion, thought, belief, judgment, estimation
"popular perceptions of old age"
•intuitive understanding and insight.
"“He wouldn't have accepted,” said my mother with unusual perception"
synonyms: insight, perceptiveness, percipience, perspicacity, understanding, sharpness, sharp-wittedness, intelligence, intuition, cleverness, incisiveness, trenchancy, astuteness, shrewdness, acuteness, acuity, discernment, sensitivity, penetration, thoughtfulness, profundity; formalperspicuity
"he talks with great perception"
THAT is the textbook definition of perception.
Take it, and make of it what you will. But make no mistake, we as humans are visual creatures by nature, so yes sight is the primary lead in to ALL perception checks. Noise is an other. So is taste. Also touch. Then there is the psychological sides. (Hey! look at that! lines up with the definition!).
That makes it as simple or complex as the depth of YOUR understanding of your awareness, which no one here can help you with. Now apply that.
THAT is the only answer you are going to get.
Why?
Reasons. This is not a study into the limitations of the human mind, nor is a doctorate in Human Anatomy needed to play (or write for it for that matter). IT IS A GAME!
Yes, the rules do not cover every thing and nor could they (so don't be more of an ass and say "why not?"..... If you can't comprehend why with being hand-held to the answer, gaming is not for you!)
What matters is the scene that the action takes place, not the whole wide world around them..... and if you insist on thinking that way. Come back next year after the GM is done rolling for the security checkpoint to get anywhere for the 25,000 NPCs in front of you. Or the GM just uses Fiat.
And the same with a scene. If the GM finds it reasonable that you get a perception check for a mage casting a spell in a dark room, so be it. IF he also says you don't So be it too. (as long as the rulings are consistently followed for all sides). That means some people in a scene will get a check for various things, others won't, all depending on how the GM feels about it and the situation.
end of story.
Now, you can come up with as many cute little situations as you want, The answer will always be the same: "As the GM commands". And it will always be the same.
Why?
Reasons.
The chief being. "That's his job". It is the GM's job to arbitrate the rules VS what the players are trying to do. This is done through the use of modifiers, fiat, logic, plot and gut reaction. No the books don't cover everything, nor could they. Go to the library, go to the Law section and look at the HUNDREDS of volumes of books just to contractual law! THAT is what you would be looking at if the books DID cover everything (or attempted to). Hundreds of volumes to buy at hundreds of dollars each. Who is going to buy that? Who is going to WRITE that?? WHO Is going to read 200 books just to play a game - let alone buy that many just to play a game?

Starting to see why I find these "yea But" arguments in general to be Pedantic.
Will the GM get it wrong from time to time? Yep. But he will get it right too.
Isn't this wrong, shouldn't he follow all the rules to the exact letter? Sure, But I bet you dollars to donuts you still won't agree with his judgements all the time even if he does. welcome to Human Interaction 101. Even simple laws like "DON'T KILL PEOPLE!!!" require lawyers (players) and Judges (GM) to determine who is right and wrong. and that's a 3 word real law! The GM's other job is to tell a story - you know, provide the action so you player are having fun? Sometimes, a rule gets in the way, so he changes it. Or he just doesn't like a rule, so he ignores it.
That's his job... Keeping the game moving forward, arbitrating rules, telling a story. Let him do it.
And if you ARE the GM, then be the GM. Make a judgement call. Move forward. Don't like a rule, change it to fit what your table needs. Just apply your changes across the board, no 1 set of rules for the players, 1 for the NPCs.
And if all you are looking for is a set number.
43.
Why?
Reasons.