Tradition. For instance, in the AV world, when a speakerphone or headset’s mic is muted, it’s common to have a red light on.

I see it more of a way to alert the user that devices might not work as expected if there is red. From the manufacturer perspective, they want to minimize the number of phonecalls that a webcam/mic isn’t working and red will more likely get user attention before someone calls

Toilet door locks indicate they’re locked with red. Similar for a lot of other types of mechanical locks.

Red light indicators for active recording is a more recent thing, mostly a consequence of red LED lights being easiest and cheapest to manufacture at scale

I think it’s more of a standard to have red indicate a disconnected state. My Thinkpad T14s has a red dot on the webcam cover, wireless peripherals like mice have a red indicator when they’re off, etc.

Edit: I also don’t think it should be confused for the “activity being used” LED indicator. The disconnected indicator is just paint, not a light.

Everyone is overcomplicating this. Red = stop. When it’s red, that means it’s off.

As for why indicators are sometimes red lights, that’s because casting light requires active effort, so if a light is on, then you can assume that the thing is at least receiving power. Red LEDs are the cheapest LEDs (or at least they used to be), which is why binary on/off indicators will sometimes use them. It has less to do with the color and more to do with the price (that’s also why older indicators are usually amber or have colored plastic filters; LEDs weren’t invented yet, so the cheapest thing was a tiny incandescent bulb with an optional color filter). Otherwise, red lights usually signal an urgent warning, error, malfunction, or some other event that requires someone to stop whatever they’re doing and give the device immediate attention.

Edit: in the case of recording specifically, people likely started using red as a recording indicator because, well, recording is important and requires your attention.

Firearms are typically red when the safety is off and white or blank/black when the safety is on.

Different technologies decided on different labels. Doesn’t always make sense.

Can you flip the switch so to speak?

Fully agreed. The highlighted color should indicate a dangerous state.

Being locked is not dangerous, you attempt to use the microphone or camera and it doesn’t work, and you remember it’s locked. So it’s a self-solving problem

I agree but the way I see it is that “red” on a gun safety means fire, and “red” on your camera switch should also mean “fire.”

Technically, the color has always indicated the opposite of the expected default.

Passive indicators on switches like this historically have used the color to signify when something is disabled, because normally you expect that thing to be enabled. Look back to old devices with mute and disable switches like the old iPhones, Palm devices, etc. and the color always signified the thing being disabled. The default state is enabled, and the switch is disabling it.

Active indicators like LEDs being used on devices to indicate things like the mic or cam being on are generally newer. But even going back to things like the red recording lights, that’s because the expected default state was off, and the indicator was showing it was in a secondary state.

The color in both cases indicates the thing being controlled is in a secondary state, but the expected default state is different in the two scenarios.