It is a minor cutscene.
After beating Level U-10, another cutscene starring the Duplicator and the Runner will begin. The Duplicator whispers to the Runner asking where she put the Planet, and that he promises not to tell anyone.
The Runner looks exasperated as she explains once again that she didn't steal it, and asks the Duplicator why else a planet might move. She then leaves to map out another tunnel and tells the Duplicator to "catch up once you realize that the Planet orbits faster than us."