Have you ever accidentally brought a bottle of shampoo or scissors? You are vastly underestimating how common it is to have a gun in some parts of the country. It is just an everyday item like anything else to some people.
Last time I flew it was almost mandatory to go through the body scanner. The time before that it was only every 10 people or so.
It's a truly amazing phenomena, no doubt part of the re-normalization of carrying guns (saw my first open carrier at Wal-Mart yesterday, that's newly allowed in Missouri as of a year or so), and the graying of America (a large majority of the people taking my Missouri required concealed carry class were older sorts).
But if you're e.g. reading this from California, New York, Massachusetts, New Jersey, Maryland, etc. vs. the 43 states with shall issue or better concealed carry regiemes, you could easily know little to nothing about it.
You're right: most people simply don't mention it. In most of the US, guns are simply not a big deal.
One clarification request: loaded if revolver, round in chamber otherwise?
Carrying requires a different mindset, and that mindset doesn't admit the sort of forgetfulness under consideration here. That's the main reason why I usually don't carry.
I can't find any stats, but I wouldn't be surprised if the number of people killed by aggressive use of shampoo was close to zero.
A friend of mine tried to bring his gun through security. He had it in an outside pocket of his duffel bag and just forgot it was there. It's just another thing that you carry around with you. It seems lost on some people that these are deadly weapons and it may be good to treat them with a modicum of extra respect. It never occurs to my friend that someone could take his bag, discover the gun, and kill someone with it, even though he never would. That it could be a kid that shoots themselves or someone else in complete innocence. My friend is indeed absent-minded but I don't think that covers it fully. It is a certain mindset, and a fairly prevalent one especially in the South.