From Bulbapedia, the community-driven Pokémon encyclopedia.
The Itemfinder is used to find items
and will activate when a tile containing an item is onscreen, giving the player an approximate location. When the player is not near a tile with a hidden item, text stating that there are no items nearby: "''Nope! ITEMFINDER isn't responding.''" If the player is near a tile with a hidden item, the item will give out four musical chimes, with text stating: "''Yes! ITEMFINDER indicates there's an item nearby.''"