Block Sensor

From the Super Mario Wiki

Block Sensor is a rare item in Mario Party DS. They cost 20 coins. It can only be found in a Grab Bag or during the final turns of a 30-turn game when a Grab Bag is also available. When used, on the next Space the player stops, a Hidden Block appears. Inside the hidden block could be anything, such as coins, a star, or even a Ztar.