Overheard Words at Konya

During a stop at Konya, Poirot steps onto the snowy platform and overhears Mary Debenham and Colonel Arbuthnot speaking in the shadows. She says, 'Not now. Not now. When it's all over. When it's behind us—then—' The words are charged with emotion and secrecy, puzzling Poirot and planting a seed of suspicion. He later recalls this moment as a key clue.