Plot twist: The real reason why Gus was at that gas station to buy a new pack of cigarettes was not because he had misplaced them but rather because he had finished smoking them. Saying that it was a metaphor was just so that he could keep cigarettes on him whenever he wanted. His cancer came back because he had started to smoke.
(I'm sorry. I know you must think I have a real issue with TFIOS. I don't. Really.)
*Read my full review of The Fault In Our Stars here: http://twentythreeowls.blogspot.sg/2014/06/the-faut-in-our-stars-honest-review.html