I suppose, I may not ask you, monsieur, what first led you to suspect the fact?" Poirot smiled and answered: "A scribbled over old envelope, and a freshly planted bed of begonias." John, I think, would have pressed his questions further, but at that moment the loud purr of a motor was audible, and we all turned to the window as it swept past. 
ALl of us were inside of a castle.