If an orange dot appears, How do I know which app is being used?

Yes, it is new, they had the green light for when the camera was activated before and now they have added an orange light for when the microphone is active or "hot". This is only when you are making a call or doing anything where you are using the microphone.

We’re you perhaps using the phone to make or receive a hone call? If so, that would do it.

Open Control Center. It will probably be the Phone app.