A Randy Mystery is a side mission in the video game, "South Park: The Fractured But Whole".
Plot details follow.
When you meet Randy, he tasks you with finding the culprit of his car keying. Continue with the story missions to naturally come across the offender: Randy himself in The Bowels of the Beast. Beat up Randy to finish this mission.
As a result, he will be able for a selfie from day two.