Yes, I don't mean to say it's impossible - just that you'd need an entirely isolated system to detect when data was flowing over that link which is physically connected in all cases and cannot be updated. I don't believe we see that in either the Alexa or Google Home, but I'd be happy to be mistaken if anyone's done a more in depth teardown of these systems.

And all of this is hinged on hoping you notice LEDs firing in the corner while you're having a conversation. Perhaps a more noticeable method should be used in cases like this. A forced "beep"/tone or something from an isolated circuit hardwired to the speakers.