There are a lot of things in these twelve stories that seem like they should be symbols. After all, when Julia Stoner's dying words are, "It was the band! The speckled band!" (Band.50), you'd be forgiven for thinking that some kind of spotted ribbon is going to have some hidden meaning to the Stoner girls, that it'll symbolize family trust or something along that line. But no, the "speckled band" is quite literal: Julia's last words indicate the snake that killed her.
Or how about the Beryl Coronet? It's the title of one of the stories and everything. Surely it must symbolize, we don't know, wealth or power or excellent facial hair or something. But in fact, it's not the coronet as a whole that matters (beyond the ruin its loss would bring on Alexander Holder). The coronet becomes almost mechanical, a tool for drawing out the greed of Sir George Burnwell and for proving the hidden worth of Arthur Holder. These are two cases that indicate something distinctive about objects in a detective story.
In a novel focusing on character development, symbolism is usually another tool for the author to indicate psychological depth. Like, if you want to show that a character is really depressed, you might spend a lot of time describing the heavy black clothes she wears. Those black clothes would become symbols of mourning, misery, and melancholy. Or if you wanted to symbolize a character's homesickness, you might draw attention to that one snow globe that says "I *Heart* Vermont!" that your character brought with her from hometown. That snow globe would then be a symbol of all of her unresolved feelings for the place she left behind. In books that really emphasize character, things become a way of saying something about the people who own them.
But detective stories don't handle objects this way. Things stop defining characters and start advancing the plot. They become tools for the storyteller to bring James Ryder to thieving (in "The Blue Carbuncle") or Irene Adler to blackmail (the photograph in "A Scandal in Bohemia"). Even an engineer's thumb can be reduced to mere proof that Victor Hatherley is telling the truth. The things in these stories are almost utilitarian (read: they provide a means to an end) – significant only as long as they are central to a Sherlock Holmes case. Once the mystery is exposed and the story unraveled, there's no magic left to the Blue Carbuncle, or to any of the numerous objects that come and go in Holmes's 221B apartment.