My iPhone make notification sounds randomly but screen doesn't turn on

I randomly notice my phone playing the classic notification sound but the screen doesn't turn on, and when I check there's no notification to be found. All the apps that have sound notifications turned on, also have banner notifications turned on. This has happened at times when no one else is around, and times when nothing could have produced a similar sound. So I know it's from my iPhone. I notice it around once every week.

[Re-Titled by Moderator]

Original Title: Notification sound but no notification banner