When Bender and Fry are in the vent, Bender switches his eyes for angled ones. He then looks around the room below, rotating them both in the same direction. However, if he did that, the eyes would have hit and the sound would have gotten Leela's attention.
Edit: She was pretending to fall asleep, so Fry and Bender take the decoy box. She was sitting on the real box.