Rex is chasing the Jeep from behind, it is visible  in the rear view mirror, even though the night is dark. Any object  following a vehicle would not be visible in the mirror unless it was  well illuminated. This was undoubtedly done on purpose for dramatic  effect, as was the goof of the OBJECTS IN MIRROR ARE CLOSER THAN  THEY APPEAR message showing on the drivers side mirror, but its  fun to note.