I recently got a Google Home, and I've been thinking about how to potentially build a "is-the-microphone-actually-recording?" device. A basic one should be possible just by watching the power draw, but that would probably trigger if it decides to download a firmware update or whatever too. I expect this has been done before?
Of course it's listening, at all times. Otherwise it would not be able to react to the trigger phrase. What is interesting is whether it sends this data anywhere, and that is probably impossible to ascertain, since it could store it for a while and tunnel it out with innocent looking traffic.
It would be interesting if it could be determined if it stores the passively obtained data at all. If one could monitor writes to memory while in passive state it might give a clue.