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?
Yeah, I also really doubted that they would learn to metalbend, and even if they did, they were so crushed together inside that box, it seemed like their chi would be blocked to bend at all, including earthbending, as if Ty Lee had appeared and chi-blocked them. And even if they did learn how, they would have probably gone to Toph's school near Yu Dao afterwards if it wasn't with their pride and honor of not being able to successfully take her back home--that would be so embarrassing to them to see her as a free young teacher and trying to learn from her, especially since Yu had been her previous earthbending teacher. Ha ha!
Yeah, it was most likely somebody or a group helping them get out.