With the Google Home Mini being released on October 18th, it was discovered that some of them were constantly recording audio and transmitting it back to Google without being intentionally activated. Artem Russakovskii from AndroidPolice said that his device had been silently sitting there and transmitting what it recorded back to the company for about a week before he noticed. Google said that this appeared to be a hardware bug in some of the Google Minis. A new firmware with version 1.28.100122 has already been rolled out to all Google Home Minis that disables this feature in order to prevent it from triggering unwanted recordings.
Source: https://www.bleepingcomputer.com/news/google/google-home-mini-caught-recording-audio-due-to-hardware-issue/