Mic sensing for specific patterns can be super low power. You could set it to speech-like patterns (audio in specific frequency ranges, above a volume limit that makes it audible, with peaks at known frequencies for speech), and turn it on only to record when such patterns are detected.

And since Meta advertises ~9 hours of moderate use (meaning voice assistants, constant phone connectivity, video recording) with gen3, plus 50 extra hours in the charging case... That's easily extrapolated to ~36 hours stand-by audio pattern watching and occasional recording.