So this is the thing… to run with the sketched scenario…
The players mention the cameras. This prompts the GM to make a ruling of some sort.
I wouldn't say 'prompt' here as it sounds like a computer program or pavlovian response to me. And to be clear, I am not saying this is how the process unfolds each time (there is going to be variability)
How does he determine what the cameras reveal? How helpful that information may be? How incomplete? And so on.
Well we are talking about a broad range of styles and games so there isn't one response. And like I said, this is a gray area. But a GM could be more or less objective about it. So it isn't like this gray area diminishes something like 'honoring prep' if you don't want it to.
I think most GMs try to figure out how probably it was the camera was there, and what it likely would have revealed if it were based on what he knows about the background of the scenario (and the is why when running mysteries, I think you start to learn to be more detailed because inevitably players start asking these kinds of questions).
There may not be a set answer. One GM may say “I make a roll to see what is discovered, lowroll is minimum info, high roll is significant info.” Another GM may handle it differently.
Yes, that is the nature of a ruling.
So… in the absence of a robust process, what principles are meant to guide the GM’s decision? What should he be keeping in mind when he makes his decision or ruling?
This is going to vary by game, group and type of campaign. There are going to be different principles for different approaches. If you want the players to be really solving the mystery, then you want more objective principals, but it may be a space in the game where people want to open up some drama potential. It is really going to vary
To me, these are questions that should likely have some answers. Things like “Honor your prep” is one I think you guys would likely agree with. After that, I’m much less certain.
Part of the problem is we are talking about objective mystery scenarios while also talking about other things. So if I am trying to run this as a mystery scenario the players can solve, personally I won't worry too too much about these gray areas, because there is still plenty of solid background information that they handle more objectively. But I would tent to lean on what feels probable and using rolls because it is an unknown. I may even ask my players what they think is a fair percentage based on what is the most likely thing
This is why I think it absolutely helps to break this stuff out into parts and look at them as steps in a process.
I am not sure breaking the process down is very helpful. You need to understand what a ruling is, and that the GM describes things and the players say what they want to do. But I would hesitate to get too prescriptive