Though the device is normally powered off by holding the left-side power button down then selecting YES to confirm, in order to save battery the ELEMNT/BOLT/ROAM will also beep, flash the LEDs, and start a 10 second coundown timer to power off automatically after 15 minutes* of inactivity (by default) when not in a workout.
*The inactivity period is customizable in the ELEMNT app's Settings page under CUSTOMIZE > Auto Shutdown while the device is connected.