Why do iphones go off without displaying notifications?

I'm at a hotel with a friend and at 12:44 tonight both my phone and hers lit up. I assumed that someone has sent a text to a group we were in or something, but when I checked my phone I didn't have any notifications. Not two minutes later the phones went off again, this time vibrating with the lighting up. Again; no notifications. I have no idea as to what could have caused this and I'm curious as to what it could mean. Any information would be greatly appreciated, thanks!

Pfbelvlx

No idea. Do you have Wi-Fi turned on? Maybe the hotel or its occupants have something to do with it. Sorry, I'm no help. Maybe ask the staff in the morning.

You can turn notifications on or off for each app individually in settings.

Scroll through the screens and look for any app with a red number showing at the top right - that's something with unread notifications:

(Though it is possible to turn of the red badge as well as the main notification banner).

It's an Apple product this is called an undocumented feature!