I meant light cone options. It's kind of a loaded question. Don't know specific options in scene or render. Really hard to say if it is a *bug*.  There are several options w/ soft shadows, lighting, light cone and render options that come together. Are you saying you did 2 identical renders in carrara and you feel one is allowing the light to pass thru the fog and one isn't? I didn't do raytrace or GI just light thru trans and got a pretty soft look. As to whether it is an exact physical simulation...well another loaded question.

No need to think outside the box....
    Just make it invisible.