Final answer:
The student's question refers to a scenario involving characters Debenham and Arbuthnot and the detective Poirot, from a literary work likely related to a mystery or detective story. Specific details cannot be given without the exact context or text.
Step-by-step explanation:
The student's question appears to address a specific dialogue or statement from a literary work, as it mentions what a character named Debenham said to another character, Arbuthnot, that lingered in the mind of the detective character Poirot.
The excerpts provided do not directly answer this question, but they give us a sense of the type of dialogue and character interaction one might find in a novel of similar style. It is likely from a work of fiction such as a mystery or novel that involves intricate character relationships and possibly a detective story element.
Without the precise context or text wherein Debenham speaks to Arbuthnot and mention of Poirot, we cannot provide a specific answer.
However, this appears to be a reference to Agatha Christie's renowned detective character, Hercule Poirot, which suggests that the student is asking about an event that takes place in one of Christie's works.
Hercule Poirot is known for being meticulously observant, and it would be characteristic of him to cling to a seemingly minor detail that others might overlook, often leading to a breakthrough in his investigations.