Help Phone vibrates/note. sound but when I turn on the screen there's no notification

[mygrapefruit]

Happened thrice past 2 hours or so. The screen is off. Suddenly the phone vibrates and makes the default notification sound (Cloud) but when I turn on the screen there's no kind of message indicating why the phone decided to pling just then.

Any clue what's up with that?

 

[mygrapefruit]

Bump? :/ It happens again right now.