Rocking Out

Rocking Out is a possible upcoming quest.

On May 21st 2008, Jagex updated the in-game skill guides with additional information. When one clicks on the thieving guide and selects the Stall option it indicates:

Level 63 Customs evidence files (after Rocking Out)

It is unknown if this was done accidentally or if Jagex purposefully placed it there as on occasion, they like to drop hints about future updates within the game itself.