From the Super Mario Wiki
Secret Island is a small secluded area introduced in New Super Mario Bros. U. It is situated between Acorn Plains and Sparkling Waters, and can only be accessed via the world map's Cannon Pipes after the game is completed. The island only houses one purple Records Toad House, which itself contains various records, such as how many coins players have collected, how many Boost Blocks were placed, and so on and so forth.
Various flags, coins, Brick Blocks, Super Mushrooms, Yoshi Eggs, and Propeller Mushrooms can be seen in the background, along with the bags which contain P-Acorns, implying that this is where the red Toads or Nabbit stash away items.
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