This morning I noticed a soft white spot on the lower left corner of my Macbook. It's not a dead pixel. I've seen those before and this is different. This spot only shows up as a light spot when the screen has a light background. On a darker screen it totally disappears. The spot is a few mm in size and. I've examined the top of the LCD case and it's smooth. The Macbook has not been dropped in anyway. Any ideas?