It might be that although the sound is inaudible to humans, it gets distorted around and inside the device, and that distorted sound is at audible frequencies t

It might be that although the sound is inaudible to humans, it gets distorted around and inside the device, and that distorted sound is at audible frequencies that the alexa microphone picks up. Something like how these ultrasonic directional speakers work: https://www.holosonics.com/what-makes-a-sound-source-directional https://www.holosonics.com/what-makes-a-sound-source-directi...