I'm not sure that there are any "secret codes" in metroid. There's an interesting analysis of the password system here <a href="https://youtu.be/wiOalxCQ1Mw" rel="nofollow">https://youtu.be/wiOalxCQ1Mw</a> and I think there's just a lot of coincidence and a permissive password system that result in "secret codes"