How did Zero and Stanley go missing in Holes?

In chapter thirty, tensions peak when Zigzag and the other boys begin bullying Stanley for allowing Zero to dig part of his hole. When things turn physical and Zigzag begins to beat Stanley, Zero intervenes and starts choking Zigzag. After the truth about Stanley teaching Zero is revealed, the Warden quizzes Zero, and Mr. Pendanski makes fun of his illiteracy, which prompts Zero to swing his shovel at the counselor and knock him unconscious. Zero immediately leaves the camp and the Warden instructs the counselors to wait for his inevitable return to Camp Green Lake.
Stanley understands that Zero's only chance of survival is to make it to God's Thumb and attempts to steal Mr. Sir's water truck to save him but ends up crashing it into a hole. Stanley then gets out of the vehicle and runs into the desert in search of Zero.
Essentially, the two boys go missing in the desert after fleeing the camp. The two boys reunite in the middle of the desert, survive on sploosh for a short time, and travel to God's Thumb, where Stanley carries Zero to the top of the mountain, and the two boys replenish themselves with onions and fresh water.

Zero first goes missing in chapter 30, after the Warden discovers that Zero is helping to dig Stanley's holes in exchange for reading lessons. The Warden subjects Zero to humiliation in front of all the other boys, and Mr. Pendanski tells Zero "digging is all you'll ever be good for." Zero knocks him out with his shovel and takes off into the desert. Stanley knows that Zero's only chance of surviving out there is to find God's Thumb. Angry and afraid for his friend, Stanley gets inspiration from the new kid Twitch and tries to hijack Mr. Sir's car to drive out and search for Zero, but he ends up crashing the car into a hole. His only choice is to run away with his empty canteen. The Warden, Mr. Pendanski, and Mr. Sir have destroyed Zero's files, so to them, he never existed. In a sense, Stanley sacrifices himself by venturing out into the desert in order to save Zero's life.