Why does my iPhone sometimes have notification sounds but nothing pops out?

It has been happening like this since last month, sometimes I heard notification sound from my phone but when I check my phone nothing pops out on the lock screen or home screen. Do you know why does it happen?

It may be because one of your applications are set not to display anything, but still make a sound. To fix this, go to settings > Notification Center and check all of the applications.

@bradley is right. I get sound notifications for my emails but I don't list them on my lock screen. Get too many. Best luck.