This is every hidden platform in the game. There are none in the intro stage, the middle stage, or the top right stage in the Japanese version.
Top left stage, health platform:
Bottom left stage, P platform:
Top right stage, US version, P platform:
Bottom right stage, health platform:
Final stage, health platform (this is right after beating the second boss):
If (and only if!) you don't get that platform, you can get this one instead, which appears a few screens later and right before the third boss: