“This would happen without the app requesting microphone access permission and without the app leaving any trace that it was listening to the microphone.”
Any app with Bluetooth access could record what you said to Siri through AirPods. No microphone prompt, no orange dot, nothing. Apple sells the permission system as the reason iPhones are private. The microphone door was locked, and a system daemon left the Bluetooth window open. Apple shipped the fix in iOS 16.1.