Can somebody explain this mystery!

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http://forums.androidcentral.com – There might be a really obvious explanation so I apologise in advance! My phone has been making notification sounds/vibrations once a day that when I look at the screen there is nothing there and nothing in the notifications pull down. Then today i was using my phone when the same thing happened but I noticed a triangle in the corner of my screen with an exclamation mark inside which disappeared almost as quick. There is no indication why this is happening or what it is for. Can anybody help me? (HowTos)