Haptic feedback
What prompts haptic feedback when the iPhone is off (screen is off, not powered down). Is it tied to an alert or notification? If so, which one? My phone has been getting the haptic 'click' sometimes when it is off. Happened multiple times today while in my pocket and when I was holding it. Did not appear to be tied to new texts or emails, so curious as to what it means.
iPhone XR, iOS 12