My guess, like that of others, is that it's a reflection of something within the vehicle. Looking closely the image seems to be a 'double', indicating the reflections came from both the inner and outer surfaces of the glass. As to what the object is I wouldn't have a clue - it could be an instrument on the car's dashboard (though I doubt that unless it's an older car) to someone's wrist watch or a reflective button on the upholstery or someone's apparel.
