Well, normally it's not really a matter of the extra being "missing." In some camera/lens systems, they shoot on standard 35mm negative stock, with a straight (non-anamorphic) lens. Thus, the camera records a 4x3 image. The image that they are actually shooting, however, is the matted ~2.3:1 area. The rest of the image recorded on the negative is not actually part of what the director intended to be part of the movie.
So even though there is stuff record on a negative somewhere, it's not really part of the movie. Which is totally different from scan-and-pan, which is actually deleting part of the image that was shown in the theater.