The Curious Case of the Dog in the Night-time (Symbol)
Gregory: "Is there any other point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."
Gregory: "The dog did nothing in the night-time."
Holmes: "That was the curious incident."
This quote refers to the fact that Sherlock can see that whoever broke into the house must be known to the dog, as otherwise we would have stirred in the night and awoken the others. It represents Sherlock's ability to see the truth in what others overlook as the mundane. It thus becomes a symbol of Sherlock's striking intellect and eye for observing what others cannot or will not.
Written Word (Motif)
Such as in the case of 'The Adventures of the Gloria Scott', where Holmes details out the events that led to his line of work, the written word through letters or writing at the scene of the crime is used as a constant motif in the Memoirs of Sherlock Holmes. In the case of the example, one letter is written to Sherlock's friend's father informing him through code that his secret is out, and another is written by the father to his son detailing out his crimes. Written word is used as a means of conveying and displaying guilt in this sense.
The Key (motif)
The idea of the key, both literal and metaphorical, appears throughout the Sherlock Holmes Memoirs. In 'The Adventure of the Crocked Man' it is both, as the missing key becomes the key to finding out who committed the crime. Most of the tales have a particular object or fact that is withheld from Sherlock, the audience, or both, before the crime and be solved or the murderer found.
Trains (Symbol)
Although Baker Street is where the most significant amount of the collection takes place, Holmes and Watson often travel vast distances out of London to solve crimes. The trains they ride on give variation to setting and character, meaning the reader does not lose interest in the crimes by confining their imagination to a small area of London. The mysteries and criminals are varied and so too must their setting. The train then becomes a symbol for the mental journey both Sherlock and the reader must take in order to solve the mysteries.
Footprints (allegory)
Holmes often becomes angry with the police who inspect crime scenes before he has the chance, as they trample footprints into the ground and cover the criminal’s footprints, destroying evidence. When a police detective informs him in one case that a tarp was put down to walk upon and preserve initial evidence Sherlock is markedly impressed. For Sherlock the footprint is both literal and allegorical, as it represents the mark the killer leaves at the scene. No matter how clean or careful they are they leave a part of themselves there, and Sherlock has the ability to find out what part it is and follow it until the mystery is solved.
