The Lost Ambassador by E. Phillips Oppenheim

During the whole of the time people had been coming and going from the restaurant, not, perhaps, in a continual stream, but still at fairly regular intervals. It seemed to me, who had watched them all with interest, that scarcely a person had entered who was not worthy of observation. I saw faces, it is true, which I had seen before at the fashionable haunts of Paris, upon the polo ground, at Longchamps, or in the Bois, yet somehow it seemed to me that they came to this place as different beings. There was a tense look in their faces, a look almost of apprehension, as they entered and passed out, -- as of people who have found their way a little further into life than their associates. Louis was right. There was something different about the place, something at which I could only dimly guess, which at that time I did not understand.

Only I realized that I watched always with a little thrill of interest whenever the hurrying forward of Monsieur Carvin indicated the arrival of a new visitor.