"Both say that they heard a shot.  One says that he reached this opening in time to glimpse Agnew past the Martian, who pretty well filled the space between the rocks; they're so big.  And then he didn't see him.  The second man says that when he got there the Martian was just exiting, simply sailed on past them and went his way - which is characteristically Martian; if he has no business with you, he simply ignores you.  With the Martian out of the way they could both see the space between the two rocks... and it was a dead end, empty.