Smile Detector

The Smile Detector is a treasure in, that can be found in the Back-and-Forth Road area. It is actually a gray, rectangular that is the same height as Olimar.

Collecting the treasure
Progress up to the White Spectralid cutscene, and enter the doorway as normal. You will then be presented with an iron block, and another doorway suspended in the air. Push the iron block under the doorway, and use the platform above the block to drop onto it. The treasure is buried right next to door’s exit, but be careful to not let any Pikmin die from the Female Shooting Spinner suspended above the treasure.

Trivia

 * The back side of this treasure, only viewable in the Treasure Log by rotating it, shows several Red Pikmin on the screen looking at the camera. This could imply that a photograph was taken of the Pikmin as the treasure was being collected, and may be an Easter egg.