Red circle near battery icon keeps coming on my iPad

The red circle (indicating that my microphone is turned on) keeps coming on.

So far, I've turned off the microphone in the Settings > privacy and in individual apps.

I've also turned off Bluetooth in case my iPad is trying to sync up.

I've also disabled Siri.

I'm not at all happy about the fact that my iPad's mic is randomly activating and there seems no easy or obvious way to get it to stop.

[Edited by Moderator]