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. 
We all heard a vehicle go by outside.
The lot of us were on a boat in the middle of the ocean.