Poirot shrugged his most Gallic shrug. “But you do not see with the eyes of Number Four, Hastings,” he said. “You talk of evidence, but what evidence have we against him? True, we have a body, but we have no proof even that the man was murdered⁠—prussic acid, when inhaled, leaves no trace. Again, we can find no one who saw anyone enter the flat during our absence, and we have found out nothing about the movements of our late friend, Mayerling⁠ ⁠…

“No, Hastings, Number Four has left no trace, and he knows it. His visit we may call a reconnaissance. Perhaps he wanted to make quite sure that Mayerling was dead, but more likely, I think, he came to see Hercule Poirot, and to have speech with the adversary whom alone he must fear.”

Poirot’s reasoning appeared to be typically egotistical, but I forbore to argue.

“And what about the inquest?” I asked. “I suppose you will explain things clearly there, and let the police have a full description of Number Four.”