Source: Google’s Nest alarm system has a microphone, unknown to consumers – CBS News
A few other notes and observations. The microphone is not even listed in the device electronic schematics. Google is a tech leader so what is the likelihood they just forgot to include this component? Note, after this was revealed, Google allowed users to opt out and turn the microphone off but in the EULA, users that turn the microphone off will no longer receive product updates (security patches, bug fixes, etc.).
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