Marshall doesn't look in his rear-view mirror. Not that he's careless about his future, but there's too much glare.
Marshall's gaze goes back along the route of his passage. Some huge cartoonist has drawn helical "zip lines" all over the concrete, the better to express the action. Motion in time become marks in space.
In the middle of the big cartoon he sees the body. It is too large to be a dog or a jackrabbit. Nor do deer wear boots.