@Wabe - yeah, I know it's not real ("Do you think that's 'air' you're breathing?") - it's just the only way I could think to describe it. ;-)

If the light actually drops in its coordinates, the test render will show that (shadows falling in a new direction, etc.). If the lighting doesn't change in the render, it's an optical illusion  of the display - or happening with the display of objects.
-Lew ;-)