The sound of silence is actually the sound of a malicious smart speaker app listening in on you
https://www.theregister.co.uk/2019/10/21/alexa_google_assistant_eavesdropping/
Google Home and Amazon Alexa can easily be hacked to eavesdrop on users or extract information by asking questions that appear to come from each smart speaker provider, according to researchers.
Both platforms can be extended by third-party developers. Such apps are called Skills for Alexa and Actions for Google Home. These are invoked by voice commands – "Alexa" or "OK Google" followed by the name for the third-party app.
