How does Doyle use foreshadowing?

"The Adventure of the Speckled Band" begins with a revealing opening sentence:
ON GLANCING OVER my notes of the seventy odd cases in which I have during the last eight years studied the methods of my friend Sherlock Holmes, I find many tragic, some comic, a large number merely strange, but none commonplace; for, working as he did rather for the love of his art than for the acquirement of wealth, he refused to associate himself with any investigation which did not tend towards the unusual, and even the fantastic.
Doyle uses Watson to get the reader intrigued in the coming story by suggesting that Sherlock Holmes "refused to associate himself with any investigation which did not tend towards the unusual, and even the fantastic." Naturally the reader assumes that the story Watson is about to narrate will be unusual and possibly even fantastic. Doyle could not say this if he were the narrator himself because it would sound like self-advertising. Watson could not even say it about himself as the putative author of the published work because it might sound boastful. The reader might be skeptical. After all, any author could claim in his opening words that the story he was about to tell was gripping, spellbinding, fantastic, or anything else. But when Watson attributes everything in the story to his friend Sherlock Holmes, who only takes cases that are interesting and challenging, the reader is easily beguiled into assuming that "The Adventure of the Speckled Band" will be interesting because it fits Holmes' specifications.
Doyle often introduces his Sherlock Holmes stories in a similar manner. He has Watson state, in so many words, that the case he is about to describe is not only weird but that it demonstrates Sherlock Holmes' remarkable powers of deduction. In the opening sentence of the story about the Speckled Band, Watson also states that Holmes works for the love of his art and doesn't especially care about money. This will help to explain why Holmes gets involved in so many cases in which there is no possibility of his receiving any remuneration. In "The Red-Headed League," for example, Jabez Wilson comes to Holmes with his petty problem because he has heard that the great detective will often work for nothing if a problem intrigues him. The same is apparently true in "The Adventure of the Speckled Band." Helen Stoner has no money to pay Holmes to help her, since her stepfather has control of all her assets. Holmes takes the case because it interests him and also because he feels some sympathy for the frightened girl.
The word "Adventure" in the title also suggests that there will be something more than mere investigation and deduction. It suggests an element of danger, either to Holmes himself or to his client. In this case the danger threatens both Holmes and his client Helen Stoner in the person of the half-mad Dr. Roylott. So the reader is promised adventure as well as an unusual and possibly fantastic story.
Doyle wrote fifty-six Sherlock Holmes stories. He was continually seeking variety in settings, characters, and crimes in order to avoid becoming formulaic, as could so easily have happened. In "The Adventure of the Scarlet Band" the story begins in London at familiar Baker Street, but all the significant action takes place out in the English countryside, where Holmes and Watson must travel by train and then by dogcart. The character of Dr. Roylott is unusual because of his personality and his specialized knowledge; and his fiendish method of committing his first crime and attempting his second crime is also unusual. The setting is a familiar English country manor, but Doyle has enlivened the place a little by adding a baboon, a cheetah, and a swamp adder, all from India.
Holmes often goes out into the country, as he does, for example, in the famous story "The Hound of the Baskervilles." In one case a swamp adder does the killing, in the other it is a gigantic hound. It was quite an achievement for Sir Arthur Conan Doyle to create fifty-six Sherlock Holmes stories as well as four Sherlock Holmes novels which all seem different in so many ways.