When Google announced its revamped Nest Aware subscriptions yesterday during Made by Google 2019, it also demonstrated one rather unexpected addition that turns ordinary smoke and CO detectors into smart ones. Nest speakers and displays will be able to listen for alarms and alert you when they hear one go off while you're out and about.
Your smart home devices will notify you via the new feed in the Google Home app when they think they hear an alarm and ask you to listen to the audio clip to confirm. You can then call emergency services right through the app. Sound detection is processed locally on your devices. You'll see a visual indicator when something is being recorded and sent to Google for review. It's also possible to review and delete any audio clip in the Home app after the fact.
Google says "Nest Aware coverage is expanding to include Nest and Google Home speakers and displays," so hopefully this means that regular first-gen Google Home speakers will be able to listen for alarms, too, and this feature won't be limited to actual Nest devices like the Nest Hub or the Nest Mini.
Sound detection "will initially be available in the U.S. only" according to Google, so it'll likely expand to other countries later.