Some other comments here seem to indicate (at least in recent models?) that it's a visual indicator light on the phone that the camera/mic is being accessed. That might be sufficient, although I might also like a small audio cue on lock (or camera engage/disengage while locked).

Unfortunately, I doubt Android will ever be able to rely on a separate visual indicator of recording, since that's another hardware component and probably hard (if not impossible) for Android to enforce.

This seems like an area where regulations would be helpful. All persistently internet connected mobile devices with cameras must have one hardware activated status LED per camera, visible to the user. Same should go for microphones actually.

The visual indicator is only in beta right now, but it will be coming to all iPhones 6s and newer in the fall.

I remember in the old Nokia days and early android days when the camera shutter sound couldn’t be disabled (peeping Tom rules) adding an indicator similar to what webcams have could be possible heck you could potentially use the flash LED on its lowest setting.

Some phones don't have such fine grained control over the flash, in my case (Galaxy Note 10+) it still is like a flashbang.

Also not an iOS developer, but I think that apps enter a “suspended” stage when you close them or press the power button to save battery life. If you look at apps’ crash logs, you can see that they are occasionally killed by a system process for using too much CPU or other resources when they’re in the background.

There are definitely constraints on background activity, but I don't know for sure whether that applies to the camera, and I don't know what its precise relation is to the screen being locked