Death in the Clouds - Agatha Christie

Poirot was seated in seat number 19 of the plane that had just taken off from Le Binet, and from that seat he was able to observe the other passengers.

Poirot shifted his gaze from one seat to another, making impressions of the passengers, and he seemed to notice everyone deeply, as always.

But what Poirot did not notice was the passenger in seat number two, behind him; On this bench rested the dead body of the murdered woman