What is Miyagi Hiding From Daniel in Cobra Kai Season 3? - Cobra Kai Theory

In the Cobra Kai season 3 snippet and trailer for season 1 and 2 that Netflix put out, we got to learn of something that Miyagi was hiding from Daniel in The Karate Kid. What could it have been? Chozen tells Daniel, "are you sure about that?" Sure about what? Theory time.