but how did they escape?

“For the problem is this: we know by what way the assassin gained admission - he entered by the door and hid himself under the bed, awaiting Mademoiselle Stangerson. But how did he leave? How did he escape? If no trap, no secret door, no hiding place, no opening of any sort is found; if the examination of the walls—even to the demolition of the pavilion—does not reveal any passage practicable—not only for a human being, but for any being whatsoever—if the ceiling shows no crack, if the floor hides no underground passage, one must really believe in the Devil.”

-Gaston Leroux, The Mystery of the Yellow Room (1907)