The Nest smoke detector microphone was never really secret. It was part of the monthly self test to determine if the alarm was working. It would send you a notification telling you it was going to sound the alarm and that it would be listening for the sound to confirm it was working.

It was listed in the features for the 2nd gen units. https://support.google.com/googlenest/answer/9229922#zippy=%...

Edit: That article isn't about the Nest Protect (smoke detector), it's about the Nest Secure, an alarm system.