Valley of Fear, Act 1, Scene 10

There was definite speculation about the meaning of the brand. From the text:

The dead man’s right arm was thrust out from his dressing gown, and exposed as high as the elbow. About halfway up the forearm was a curious brown design, a triangle inside a circle, standing out in vivid relief upon the lard-coloured skin.
“It’s not tattooed,” said the doctor, peering through his glasses. “I never saw anything like it. The man has been branded at some time as they brand cattle. What is the meaning of this?”
“I don’t profess to know the meaning of it,” said Cecil Barker; “but I have seen the mark on Douglas many times this last ten years.”
“And so have I,” said the butler. “Many a time when the master has rolled up his sleeves I have noticed that very mark. I’ve often wondered what it could be.”
“Then it has nothing to do with the crime, anyhow,” said the sergeant. “But it’s a rum thing all the same. Everything about this case is rum. Well, what is it now?”