I'll mention 3 things (and don't ask me how I know):
When your iphone is powered off, find my phone still works. Phone reserves a small amount to ping back to apple. Whether apps can hook into this service is another question.
Even with the phone off and battery ripped out, the phone can still be used as a passive microphone in the room.
I'll mention 3 things (and don't ask me how I know):