What does this quote mean? "The superintendent, a bundle of keys hanging from his belt, was standing at the door. Rodent and superintendent vanished into the lobby, only to emerge moments later, animal first."

I think this quote refers to A guard and a prisoner. The guard is the superintendent with the keys to the cell. He is standing by the prisoners door, and let the prisoner out into the lobby. I think the last part means that the prisoner got loose because he was first out the door.