The technology giant said an “error” meant the microphone was not included in the specifications for the device, but it was “never intended to be a secret”.
The hardware feature came to light after Google announced a software update to the Nest Secure system would enable it to use its voice-activated helper, Google Assistant, which is powered by artificial intelligence to answer queries and commands.
Until then, the product web page for the device did not say it contained a microphone. It has since been updated.
A Google spokesman said: “The on-device microphone was never intended to be a secret and should have been listed in the tech specs. That was an error on our part. The microphone has never been on and is only activated when users specifically enable the option.
The device is part of what Google calls its Nest alarm system starter pack, providing users with motion sensors and a keypad to create a bespoke home security set-up.
Silkie Carlo, director of UK-based privacy campaign group Big Brother Watch, said the incident would do little to help public trust around smart home devices.
“It is hard to believe Google cares about people’s privacy after selling a security product with a secret microphone in it,” she said.
“This appears to be deceptive rather than a ‘mistake’, which is incredibly damaging for public trust in Google.
“Many of our worries about smart home devices appear to be proving true.
“Google should be held to account for wrongly advertising this product.”