Hi, I have been noticing that when I power on my iPhone 4S I see what looks like a dead pixel flickering briefly on the screen. It happens when the screen goes to that silver apple logo w/black background. It happens very briefly (1sec or less) and it seems to appear on random locations. I tested my screen for dead pixels but I couldn't find any. It seems it only occurs when powering it on. Has anyone noticed this on theirs?