>>719755124
Unmarked secret: Level design Pattern recognition
But it's main contribution is the usage of specific repeated patterns used in different contexts.
Picture for example: a dead-end with a certain white block texture.
This is not stricly "putting a secret in a negative space design" that is successfully used in a lot of modern games, because Metroid 2 has a lot of useless dead-end and a lot of unused negative space.
This counts on the player learning the patterns he encouters and recognise them even if not explicitly taught.
This is still widely used in Nintendo games. Wall after a pipe? hidden block in Mario. Nine bushes? Hidden entrance in Zelda.