Solution to Mystery Forshadowed in Opening Scene
I have watched this movie many times but only recently noticed that the solution to the mystery is foreshadowed in the opening scene with Chalmers, the insurance industry representative.
As Chalmers tells Holmes and Watson about the strange events at Drearcliff House, Holmes nervously moves about in search of tobacco, and eventually says, "Isn't there any tobacco about this place?".
Later in the film, it is Captain Simpson's urgent need for tobacco that causes the entire insurance scam to unravel.
I thought this was nicely done by the writers and Rathbone is so smooth he doesn't draw too much attention to what he's doing.