What is the element of suspense and suspicion in "Dusk" by Saki?
The young man turned to him with a look of disarming frankness which put him instantly on his guard.
Gortsby is not an easy mark. He is a city dweller and, as the story indicates, he is a great observer of people. He suspects that this young stranger is a con artist and is going to try to tap him for money. Why else would a complete stranger sit beside him on a park bench and start putting on an act?
What is unusual in this story is the business with the soap. The reader is left wondering whether it was the young man or the elderly gentleman who was the con man. The elderly gentleman who had been sitting on the bench before was obviously the one who had lost the soap--but maybe he dropped it deliberately with the intention of coming back a bit later and telling Gorsby pretty much the same story that Gortsby had heard from the young man.
This could be more than a coincidence. It could be that this particular story was being used by a number of con men at that time because it was effective. Nowadays, one of the stories we often get from small-time con men is that they need money to make a phone call. Another is that they ran out of gas and need enough money to buy one gallon. But stories get worn out from overuse and have to be replaced with new inventions.
It seems quite possible that both the young man and the elderly gentleman were working the same scam--only the elderly gentleman was more experienced at his profession and had actually provided the cake of soap to show to the mark if he asked for proof to substantiate the story. The young man--assuming he is a con artist--will probably start carrying a bar of soap in the future. The elderly gentleman has a nice trick: he drops the bar of soap near the bench and then has an excuse to start up a conversation when he comes back to retrieve it. This elderly gentleman has probably tried his sob story before and has been asked about the bar of soap he claims to have left his hotel to purchase.
Really good panhandlers can make a lot of money if they pick the right locations. Stephen King writes about a man who was making hundreds of dollars a day panhandling at a perfect location in Lower Manhattan. This story is in King's book Hearts in Atlantis.
Why do two people pick on Gortsby within such a short time? It is probably because he is fairly well dressed and his body language makes him seem open to conversation with strangers. He is also fairly young--and young people are generally much easier victims of all sorts of predators!