Try any voice recording app for a few hours, now use the facebook app for the same number of hours. The impact on battery life of a mic actively recording alone is very noticeable, so noticeable that your phone has a special chip just to recognize patterns similar to “hey siri”.
It would not need to record high quality audio and could maybe even take advantage of that same chip? Just thinking out loud here - smaller, crappier audio would also be easier to send back unnoticed (or instead of even recording audio it could be transcribing on the fly to a text file using something super basic and easy with low accuracy)