After Toph first learned metalbending, she escaped from the metal box and metalbent Yu and Xin Fu in there. Xin Fu stated, "I'm gonna be trapped in here with you forever, aren't I?" in the episode, but somehow they escaped, which is implied in The Rift Part 3 in the first pages by Toph's dad that the 2 men gave up their search for her. How did they escape from the box? Did they learn how to metalbend? Or did someone or a group find them and helped to break the metal and get them out?
Will we ever know?