Keep in mind, in my job, I am at the other end of the pipeline in the film process. I receive already-processed films to present in the theater. I don't do the same things they do but similar.
I receive a can of reels to present in the theater. I open the can, inspect the 2,000 ft. reels for damage, count and informally inventory them to be sure I have a complete movie in good condition.
Then I start with reel no. 1 of the movie, take off the leader and inspect the head of the film. If it needs to be rewound, I mount it on the bench winder and start the motor. Once the condition and orientation of the reel are verified, I splice the first frame of that reel to the end of the projection leader and spool on, either to the platter system or to a large, 6,000 ft. reel for changeover operation.
This routine is repeated for each reel of film until the last one is reached. At this juncture, I have to search the last reel in order to find the point where the credits begin. That is where the foil cue is placed on the edge of the film to trip the sensor that controls the automation system. This will fade the auditorium lights to half brightness. There is another cue at the end of the credits which triggers the projector's shutdown sequence and bring the lights to full brightness. Up to that point, I would not have spent much time looking at the images on the film at all.
At the beginning of each reel, I need to find the first frame of action and determine where the splice needs to be made. I find the last frame of action on the tail of the previous reel and make the join. I have actually only looked at a few feet at the ends of each reel. Even the, my attention is focused on the film, the sprocket holes, the frame lines and my splicing equipment. I see the images on the film only in the context of making a clean, in-frame splice.
When each reel is handled, I do look at the condition of the film images to see whether the emulsion is scratched, the image is clear and presentable and that the film support is fit to project. (e.g. Broken sprocket holes, damaged or creased film, etc.) I have seen lab defects like fogged film, misaligned frames and other things. I do look for them but the film is often running at substantial speed on the bench winder.
Here's the key: I still don't spend much time looking at the actual images. The film is going by too fast to see each frame. Instead I look for visual cues on the moving film.
If the film was fogged at the lab, the color of the moving band of film will suddenly change. If the negative went out of alignment in the printing machine, the image will appear to "zig-zag" as it goes by. If there are scratches on the emulsion, the sheen of the surface as the light reflects off it will change. I lightly run the edges of the film between my (gloved) fingers to feel for defects. You can feel your gloves catch on defects as the film goes by. If there are any defects or splices that shouldn't be there, I will stop and inspect and make repairs as necessary. (Or call for a replacement reel.)
Those would be the only times when I actually look at the images on film. Even then, I hardly remember any of the pictures I saw, even if I used a magnifier.
(I keep an old projection lens on the bench and use it as a loupe if I need to inspect film more closely.)
In fifteen years of operating movie theaters, I there are only a few times when I have remembered what I have seen on film before it was projected for a technical pre-screening or for an audience. Since 99% of the film I handled has been release prints for general public viewing it's already been blessed by the MPAA or European review boards as safe for public viewing. But for the occasional nude scene, I'm not going to find anything outrageous on my film. Even then, I still don't remember much of what I saw. There's just too much information for my brain to recall.
Now, take my experience and move backwards, up the pipeline, to the development labs. I don't think it makes much difference whether we're talking about a Hollywood film lab or a consumer film lab. I think the operations are similar enough that the differences don't matter.
Much of the time, lab workers would either be working with exposed, undeveloped film, still inside the cartridge or box. They have to be in the dark when they remove the film from its container or they'd have to be working with their hands in some sort of "dark box." Up to the time when the film goes into the developing machine, no images are visible and any images that are visible can't be viewed in darkness. When the film comes out of the machine, it is spooled up for packaging and delivery.
I believe that the big Hollywood labs have machines to grade film and detect developing defects/problems. I don't know much about them but I am told that they are some kind of densitometer or color analyzer, probably similar to the equipment that photographers use in their work but only automated. (Others who know more about this, please fill in the blanks.) If the machine detects a problem, it is supposed to be kicked out for closer inspection by a technician. This is probably the first time when film is likely to be viewed by a person.
After that, I would absolutely expect there to be a human at the end of the processing line who inspects samples of each film that comes through. It's only sensible.
But, look at his job and compare it to mine. He would be sitting at his workbench, looking at miles and miles of film as it spools by, inspecting the first few feet or the last few feet of any reel, checking for defects that "sneaked" by the automated machines, stopping the film only when he sees a problem. Let's just guess that he might look at 100 reels of film during his shift. He might stop the film, maybe a dozen times, maybe twenty times, to look at the film. If he works five days a week, fifty weeks per year, that would be 25,000 reels of film to look at. He might have inspected 5,000 or more images in that year.
Do you think a guy like that would remember anything of what he saw? Even if his employer demands that he be vigilant and keep an eye out for any pornographic or excessively violent images, do you think a guy doing his "daily grind" would see even half of the potentially "bad" stuff? Out of that, only a handful of what he saw would warrant closer scrutiny. Out of that, only a very few might require the involvement of the police.
I'm willing to be that even a "career man" who has worked his way up from the mail room, through the film labs and into management will only be able to recall a miniscule fraction of what he saw on film and might remember a handful of incidents where he found "bad" images on film. I'll bet you a nickel that he could count them on one hand.