As Watson gazes up at Holmes's room at Baker Street, he can immediately tell from his friend's body language that he is in the middle of solving a mystery. Through visual imagery and an idiom, Watson explains how he knows that Holmes is "at work again."
He was pacing the room swiftly, eagerly, with his head sunk upon his chest and his hands clasped behind him. To me, who knew his every mood and habit, his attitude and manner told their own story. He was at work again. He had risen out of his drug-created dreams and was hot upon the scent of some new problem.
In this passage, Doyle captures the appearance and movements of someone who is deep in thought. Holmes is not pensive in stillness, however, but rather paces around the room. This imagery contributes to his character development, as it shows him to be a restless and engrossed person.
Doyle, through Watson, also relies on an idiom for Holmes's character development. If someone is "hot upon the scent" of something, they are in pursuit of a clue or on the verge of solving a problem. This idiom, which signals an approaching discovery, is based on a canine metaphor. Dogs have a keen sense of smell, and often use this sense to pursue a track. Through the idiom, which is often used to describe investigators and detectives, Doyle implicitly compares Holmes to an eager dog. The imagery and idiom combine to introduce Holmes as a committed, restless detective.
In the story's third part, Holmes finds Irene Adler's letter, the text of which Watson embeds into the body of the short story. As she goes through her process of reasoning, she uses an idiom to explain her and her husband's departure for Europe:
We both thought the best resource was flight, when pursued by so formidable an antagonist; so you will find the nest empty when you call to-morrow.
Even back in the late 19th century, "empty nest" was an idiom referring to a household in which the children have grown up and moved out. In current usage, "empty nesters" are parents who have been left behind by their grown children.
By using this idiom, Irene Adler subtly plays with the power she has over the unsuspecting Holmes. As an idiom, empty nest has a slightly negative connotation, suggesting loneliness and a lack of purpose. Parents whose children have moved out sometimes don't know what to do with the surplus time and energy they suddenly have on their hands; the emptiness is, at least stereotypically, thought to result in lethargy and melancholy. When Irene Adler tells Holmes that he "will find the nest empty" when he calls, she poses herself as a dynamic young adult with her life ahead of her and Holmes as a despairing middle-aged parent robbed of purpose.
By way of this idiom, Irene Adler punctuates Holmes's vulnerability in a literary way. She states earlier in the sentence that she respects Holmes as an antagonist, which is likely true. Nevertheless, she also shows that she has triumphed over his perception and wit. Her writing style functions in line with her overall approach: clever and discreet, she maintains the upper hand without anyone—particularly not someone like Holmes—suspecting much of her. After they finish reading the elegantly written letter, the King of Bohemia applauds her. The shock over having been beaten leaves Holmes with a deep sense of admiration for the woman. This is why he asks the king to let him keep Irene Adler's photograph. For Holmes, it serves as a memento of the time he unexpectedly met his intellectual and cunning match.
