Been there
My laptop has been doing this for years, when I press the screen switch (from int to ext) an image is still displayed on the LCD, albeit very faint and only visible from certain angles. It's not just burn either, the image does change as it should but according to the instructions and diagnostic no power is flowing to the screen! Magic!
(Note: I obviously do not believe the diagnostic tools supplied with the laptop and am fully aware that this is not "spiderwebs and magic")