A little white light shining through windows

It looks like a pinpoint of light shining through any opened window, always at the same place (it doesn't move if I move the window). It looks like a screen-related hardware problem, only I don't think it's a hardware problem because if no window is opened it doesn't show. I can change the background picture, it doesn't show. Any opened window covering this place, whatever the program, it shows. An opened window not covering this place, it doesn't show. It doesn't appear on screenshots. The only fix I can think off would be to reinstall Lubuntu, but this seems like overkill for a small problem like that. Plus, I don't just want it to simply go away, I'm curious about what could possibly cause this?