In this article, mention is made of a secret room in Metroid where unused sprites were stored. The article doesn’t provide any cites, and a Google search turned up empty. Does anyone know what this is referring to, and if so, how do you get there?
This is a really poorly formatted FAQ, but it should tell you what you need to know. There’s another guide, but I only glanced and I’m not sure they’re talking about the same thing.