The doors are held on by two metal screws in the rear and 3 up front. One missing clip, even if one is missing won’t be a big deal. In all likelihood they probably put the door panel in the trunk while working on it and that came off. If it was a fresh clip there would be a white tip on it because that part of the clip snaps and stays in the door panel and the black part comes out with the panel. You can look around the the door panel and see if there’s any parts that don’t look flush, that could be where there would be a missing clip, but if there’s no rattles or noise coming from it then I would leave it be.