Perhaps this would be a good place to try to explain exactly what the young man claims happened to him, since it seems pretty odd for a person to lose a hotel.
The young man says he "came up this afternoon." He does not say from where. He meant to stay at the Patagonian Hotel in Berkshire Square, presumably a first-class hotel in a good neighborhood.
"When I got there I found it had been pulled down some weeks ago and a cinema theatre run up on the site."
How could a big hotel be torn down and a theater built on the site in a matter of weeks? This may be a clue intended to alert the reader that the stranger's story is a hoax.
The young man says he went to another hotel. He sent a letter to relatives giving them his new address and then went out to buy a cake of soap because he hates using hotel soap. This is a nice touch because it characterizes him as a high-class gentleman who has to have the best soap.
"Then I strolled about a bit, had a drink at a bar and looked at the shops, and when I came to turn my steps back to the hotel I suddenly realised that I didn't remember its name or even what street it was in."
He must have left his money in his hotel room and only came outside with about a shilling. He spent all but twopence on the soap and the drink. At least this is what he tells Gortsby.
Saki strengthens the credibility of the young man's story by having Gortsby reply:
"I remember doing exactly the same thing once in a foreign capital, and on that occasion there were two of us, which made it more remarkable. Luckily we remembered that the hotel was on a sort of canal, and when we struck the canal we were able to find our way back to the hotel."
Maybe the young man made up such a complex story in order to interest Gortsby and draw him in. Maybe it is just the weirdness of the story that makes it believable. Most of us have had the experience of "losing" something if we live in a big city. Most frequently we can't find our car in a big parking lot or multi-story parking structure.
As for the theme of Saki's story, it would seem to be the infinite duplicity of humanity and the way that some clever tricksters have learned to take advantage of other people's good nature. The title "Dusk" suggests that it is hard to see people as they truly are. In a big city almost everybody is a stranger. It is hard enough to cling to one's own identity. People in a big city can change identities if they decide to do so. The young man is never given a name. Neither is the "elderly gentleman," who could be a confidence trickster too.
Saki's story resembles Nathaniel Hawthorne's "Wakefield," a story in which a middle-aged man on a mere whim leaves his wife and his home and takes up a lodging just blocks away, where he remains an anonymous non-person for many years before deciding to return home to his wife, who believes she is a widow. Hawthorne's story is also set in London. It is intended to show how ephemeral people's identities actually are.
Even the people we think we know may not be who they say they are. Norman Gortsby might not have given the young man a guinea if he hadn't found that cake of soap by the bench. He felt gratified that the stranger was actually telling the truth and wasn't just another of the many imposters in the world--although it turned out that the young stranger really was just another imposter. Or was it the elderly gentleman who was the imposter? Or both of them?