It's probably worth noting that there are "red light cameras" that are there to give people tickets and then there are "traffic cameras" that are there to either control the lights or for security.
The light control and security cameras are usually mounted on the overhead traffic light poles, smaller, security camera looking things, those are obviously pretty moot to mark.
The "red light cameras" are usually mounted several yards behind the junction itself, so that they can capture the plates as the cars drive by. They are usually larger, in a box and are on their own poles.
Several states in the US require appropriate signs to warn drivers of their presence but not all.
In certain states, like in MD and AZ, I've seen mobile ones at construction sites or just on the side of the road, mounted to retractable booms on a trailer or on top of an SUV squad car. Those are often not marked in advance by signs.