The head of the Google Hardware division strongly suggests that guests be notified at home when a Google Home or Amazon Echo listens.
Rick Osterloh is the senior vice president of Google for his hardware. Osterloh has now revealed in an interview with the BBC that he would point out his guests to active smart speakers in his own home. Osterloh acknowledged that this question had taken him by surprise and he had not thought about it that way. But one must indeed take into account the interests of all persons who are within reach of the devices. Therefore, owners of such a smart speaker should explicitly point out their visitors who come into the receiving area of such a device.
Osterloh also says that the devices themselves should be monitoring. At least by LED make the echo and Google Home / Nest already. Google Nest cameras also use LEDs to show when capturing pictures or videos.
In order to avoid the privacy issues associated with home visitors, it may be a good idea to simply unplug the smart speakers as soon as you visit, unless you need the Smart Speaker features for the guests, such as playing Music. Alternatively, you can also press the microphone-off button on the devices. Then they still play music, but no longer listen to the wake-up call. Such mechanical off buttons are also available for some cameras, for example at Echo Show 5 and Echo Show 8 or at the Bosch 360-degree camera.