From the Super Mario Wiki
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“Sometimes if you jump at the right places, a treasure box will pop out of thin air. Anything like this happen to you? These treasure boxes are known as “Surprise Boxes”, because they appear out of nowhere.”
A Surprise Box is an invisible Treasure Box in Super Mario RPG: Legend of the Seven Stars. Mario learns about Surprise Boxes from an old Toad living in the basement of the Mushroom Kingdom Item Shop. By jumping, Mario can discover the location of the invisible treasures. A friendly Chester in Monstro Town provides Mario the number of Surprise Boxes to be found in the game. Later in the game, Mario can collect and equip an Accessory from Croco, the Signal Ring, which makes a noise whenever he's in an area or room with a Surprise Box.
For a listing of all the Surprise Boxes in the same, see: List of Surprise Boxes in Super Mario RPG: Legend of the Seven Stars
The concept of hidden containers are carried over with Hidden Block in the Paper Mario and Mario & Luigi games.