Several times a day for the past several weeks, I'm noticing my phone briefly vibrating as though I've received a notification of some sort. However, upon checking, I have no new notifications of any kind. 

Per the suggestion in [How do I know which App is sending Vibrate-only Notification?](https://apple.stackexchange.com/questions/128636/how-do-i-know-which-app-is-sending-vibrate-only-notification), I've triple checked my notifications, and any app which has sound enabled (and therefore vibration when phone is muted) should also either badge the icon or appear in history or the lock screen. Neither is occurring. 

Help?! As someone who disables the vast majority of notifications so I can stay focused during the day, this is driving me crazy!