From the Super Mario Wiki
When the player decides to stop here, he or she will meet Thwomp, who has just been robbed of forty coins. His quest, Weeping Thwomp, is a mystery where Shroomlock comes in to help. After Thwomp gives him the details, Shroomlock asks the player to search around the house for some clues. When the player is done, Shroomlock will ask which clue points to where the thief might be. However, in the end, it is revealed that there was no robbery, since the house's door is too small for Thwomp to leave. Solving the case will reward the player with the Gaddget Eye Exam.