It looks like there's also a board on the floor between the legs that quite obviously is attached to and moves with the desk, colored to blend into the floor. It's probably there just to help stabilize the prop desk. I don't know if I'd include it in your model or not, depending on whether you want to go with accuracy or intent in the model.
I think that's just a shadow of the desk being cast by the light from overhead.
I think B.J. may be right, Greg. Several other shots seem to support what he's saying. This one, for example: