Does a smart speaker hear everything?
Answer first
It listens continuously for a wake word. When it thinks it heard a command, including by mistake, speech may be processed and stored according to the service settings.
A smart speaker listens for its wake word so that it can start processing a command. The exact implementation and storage settings vary by manufacturer.
Google documents that Assistant can activate unintentionally. If Voice and Audio Activity is on, Assistant audio recordings may appear in the Google Account activity history.
Check your own service’s history for saved audio and choose the retention period yourself. Not all services provide the same controls.
If the device has a physical microphone switch, use it in a room where you do not want the speaker listening for a wake word. Do not place a speaker where confidential conversations normally happen.