Stochastically speaking, there is at least a ninety-nine percent probability that the young grifter was lying about his whole hard-luck story, including the cake of soap. He tells Norman Gortsby he went out to buy a cake of soap because he doesn't like hotel soap and then couldn't find his hotel. He had left all his money in his room except for a shilling he used to buy the soap and have a drink at a pub. Gortsby apparently is used to sitting on a park bench at around this time of evening. He has heard dozens of hard-luck stories and has become somewhat hardened and skeptical. He listens to the young man and then:
"Of course," said Gortsby slowly, "the weak point of your story is that you can't produce the soap."
This is pretty convincing proof that the young man is lying. His only explanation would have to be that he must have lost it. Gortsby is not impressed--and neither is the reader.
"To lose an hotel and a cake of soap on one afternoon suggests wilful carelessness," said Gortsby, but the young man scarcely waited to hear the end of the remark.
That is the crux of the matter. How can this swindler expect anyone to believe he has lost both his hotel and his cake of soap? When Gortsby gets up to leave and accidentally spots a wrapped cake of soap on the ground near the bench, he naturally thinks it must belong to the young man and that the young man had been telling the truth. But after Gortsby has made the bad mistake of chasing the other man down and handing him a sovereign plus a cake of soap, he finds out that the soap actually belonged to an elderly gentleman who had been sitting beside him on the park bench a short time earlier.
The young grifter's behavior when Gortsby gives him the money and the soap also enhances the probability that his whole tale was a lie.
"Lucky thing your finding it," said the youth, and then, with a catch in his voice, he blurted out a word or two of thanks and fled headlong in the direction of Knightsbridge.
The catch in his voice and his headlong flight show he is both guilty and frightened. He can't understand why anyone should hand him a cake of soap. He senses it might be some kind of police trap. He is probably afraid to accept either the money or the soap, since they could be used as evidence against him if he were arrested on a bunco charge. He probably doesn't think that Gortsby is a plainclothes cop; more likely he suspects that Gortsby reported him to a cop and that the cop is watching their meeting from some vantage point. That would explain why he "fled." He is a con man but not a very good one. He has probably just thought of this scam and is trying it out for the first time. If he keeps at it, he will be sure to keep a cake of soap in his pocket as proof of his story--but he will probably try his next trick elsewhere and will stay away from Hyde Park for a while.