I can say with a high degree of confidence that the passenger door is open in that shot. The angle of the window line is all wrong, implying that the left side is coming toward the camera. If the door is open when the rest of them are seated, that follows that someone may holding it open. Maybe that's a faulty premise I'm tossing out, but it'd be weird to have everyone seated in the car ready to go with the passenger door ajar.
I think we're seeing Grace lean over Kim's shoulder, and the zipper we see is that of a left-side chest pocket diagonal zip on the fourth person -- someone crouching slightly to get a view unobstructed by the ajar door, hence the elbow not being visible.
I've done this kind of Photoshopping for work before when time was tight -- removing just enough of a person or object so the viewer isn't distracted or the brain is primed to assign that portion to something else. Makes me wonder if Jason Reitman's cryptic post yesterday about a "ghost sighting" refers to the disembodied arm I'm seeing, or a cartoon Mooglie that could appear in the print version of the article (although all things being equal, probably the latter).
I've said my piece and I'll stop litigating it before people roll their eyes so hard they have to see an optometrist. We'll see further down the road if I'm right or my seeing things as the result of apophenia.
(dooooo bow bow -- chick -- chicka chickahhh)