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Maybe the paint ran dry in the vessel containing the red oxide primer and your car was one that was just the first or second that made it through that station before someone noticed the issue. If so, I'd imagine that the assembly line would not be stopped for someone to re-fill the container (100's of gallons?) which would take quite some time to fill.
Since the picture is on the outboard rear frame rail (think that is what I'm seeing) its more likely IMHO that this is from the coating of the outer body (in this case the lower edge of the rear pinch weld or through the wheel well opening towards the rear of the car) rather than from below unless there are more findings or clues
....................EDIT: Re-reading your post this morning and now understand what you were saying. You're of the opinion that the primer showing was from the car body being primered around that area.