Notification settings are relatively complex on a modern iPhone or iPad. With an app sounds can be played or the phone can be vibrated without visible notifications are displayed. Here's how to find the culprit.
Phantom Vibrations (and Noise)
Note that phantom vibrations are a common phenomenon. Many people have felt that their iPhone is vibrating in their pockets, just to pull it out and find that it has not vibrated at all. Especially in noisy places, phantom sounds can occur. Did the notification sound on your phone really play? Maybe that was just another sound in the middle of the noise ̵
But that's not the only problem. Your iPhone may have vibrated or played a sound even if you do not see any notifications on your notification center or lock screen when you check this.
An app has invisible notifications that play sounds.
Apps can have invisible notifications that vibrate your phone or play off your notification sound.
To verify this, go to Settings> Notifications. When an app is set to "Sounds" without a banner, a notification sound will play without displaying any visible notifications. If an app is set to "Sounds" without a banner but with "Badges", a red notification card will be displayed with a counter for new items in the app. A sound will sound when the badge is raised, but no visible notification will be displayed.
Scroll down the list and look for such devious apps. If you see one or more sounds-only settings, they're probably the cause of your iPhone's mysterious beeps and vibrations.
If you find such an app, tap it and then choose what you want to do. For example, you can completely disable notifications for this app by enabling the "Allow notifications" option under "Notifications" or enabling visible notification banners.
A notification has been displayed and has disappeared  Your phone may ring and only wake up if the notification disappears a few moments later.
This is not too common, but it happens. The same app that sent a notification to your phone can "send" it and later delete it from your phone. You can see this in action when using multi-platform messaging apps. If you see an unread message on another device, the app will often delete notification of that message from your iPhone. After all, you've already seen it on another platform. You do not have to see it anymore.
Some messaging apps also allow someone to delete a message sent to you. Depending on how the message service is programmed, the app may also delete the notification from your iPhone or iPad. In other words, if someone sends you a message and deletes it, your iPhone may beeping or humming, but the notification may disappear before you view it. Deleting messages does not automatically delete the notification associated with the message. There is a good chance that the vibration in your pocket was just a phantom vibration and was not associated with a notification that has since disappeared.
On an Android phone, it is possible to check the notification history to see if this has occurred, and to display the text of the notification. However, Apple's iOS operating system does not provide such a feature. On an iPhone, you can only determine which app sends and deletes notifications when you accidentally look at the screen when the notification appears and then disappears.