Back To The Future passenger gaffe

Here on SFX we were watching one of our favourite movies, 1985's Back To The Future, when we spotted what appears to be two figures in the DeLorean. Marty is supposed to be travelling back to the future by himself (indeed, we've just seen Doc Brown by the side of the road connecting up the electrical cable).

Are we seeing things? A trick of the light? Was this an alternative shot where there were two people in the car, which got kept in by mistake? If you've spotted this yourself and have an explanation, or if you've read about it before (it might be a well-known gaffe we just haven't come across) do let us know...