One thing I assume with respect to SD is that I'm never
alone, but that doesn't mean anyone cares what I do/what happens to me. For a cyberpunk dystopian future with cameras, microphones, and guttersnipes waiting to rat someone out for flash at the first opportunity, it's reasonable to assume that someone
. Plus of course sneaky people, cyberware, etc.
As for ambient population (such as NPCs) a single room's size can vary greatly. You've got tiny elevators and hotel cubes, as well as full city streets/blocks crowded with people. If NPCs don't seem to notice/act on something happening "right in front of them" I usually assume they may have noticed it, but crowded noisy bars, clubs, and city streets, they could be doing something else or off to the side dealing with their own things.
Alternatively, they might just not want to get involved because it's not their biz, or it'd be a hassle. They could be high or going through withdrawals, or lost in their own head and react too slowly to do anything.
This is just my personal take on why things that happen in a public place with no PCs or NPCs still get out, as well as why some NPCs might not react to things immediately. Not to mention the concept of "diffusion of responsibility" which is essentially "Well someone should help but there's lots of people around so let one of them do it" as made famous by the real life murder of Kitty Genovese (though it's since come to light in a documentary her brother did that people did phone police who simply didn't care because of the neighborhood).
In general though if it happened, it happened and someone might have seen something, but that doesn't mean they're going to actually do anything about it.
GMs can choose to leak occurrences that no PC or NPC technically saw, but the ambient population would have, but also sometimes folks just don't care or don't want to get involved. Exactly why folks should @note their secret nefarious deeds "nobody" witnessed.
Also, PCs and NPCs can and do lie/have wrong information.