Some 4th-gen Echo speakers can detect motion

Amazon has started rolling out a new feature to the fourth-generation Echo and Echo Dot to detect if someone is in the room. It now emits an "inaudible ultrasound wave," so you can enable features like turning connected devices on and off when you enter a room. 

You can enable and disable this feature through the Amazon Alexa app. It's accessible through the Motion Detection setting in the app. The app is also where you can set up occupancy routines that take advantage of this new function. It can turn lights on and off again depending on whether it detects someone. Or it can play music when motion is detected near an Echo device during a set time and then shut down once you've left.

This function works similar to the motion-sensing feature on Echo Show devices, but those rely on the camera, while this uses the ultrasound waves to detect motion. As The Verge pointed out, this feature furthers Amazon's vision of an ambient smart home, where your devices automatically respond to you with hardly any prompts from you.