I run into this issue all the time and have been wondering what is the cause and why a 'shadow' from the light still remains when the visibility settings for a light are set to off.
Is it possible that the shadow is actually AO from the geometry of the plane, or geometry that is emitting light? I ask this because I had all the visibility settings on a light turned off and the light was clipping some geometry in my scene and it was still casting what seemed like AO on the clipped geometry...