Sir Bernard Dodge and Hercule Poirot in “Unfinished Business”

Seems I may owe you an apology, M. Poirot.

Hercule Poirot

No, no, Sir Bernard. You were cleverly misled.

How did you get onto it?

Hercule Poirot

Whenever the occasion arises, Sir Bernard, to look into someone's address book, always first look under the letter "X" because that is where the secrets are kept. In the address book of Monsieur Egan which I found under his pillow, under the letter "X," there was only one number. No name. Just "X." It was a Mayfair number, which I thought strange. But when I pretended to use the telephone of Commander Daniels, I noticed that his number was the same as that in the address book. An interesting connection, n'est-ce pas?