When Helen Stoner first visits Sherlock Holmes, she describes sounds that she has been hearing in the night:
during the last few nights I have always, about three in the morning, heard a low, clear whistle. . . .
As I opened my door I seemed to hear a low whistle, such as my sister described, and a few moments later a clanging sound, as if a mass of metal had fallen.
Holmes mentally files away this information and will retrieve it when he is staying at Stoke Manor during his active investigation.
In the same initial interview, Helen Stoner tells Holmes that, because of some renovations to Stoke Manor,
I have had to move into the chamber in which my sister died, and to sleep in the very bed in which she slept.
These are two clues that Holmes uses to determine that the death of Miss Stoner's sister was a murder. Holmes also deduces that these strange occurrences figure prominently in the imminent threat to Miss Stoner's life. The sounds are significant, as is the location of her new bedchamber, and Holmes recognizes that right away. He has only to determine the particulars.
When he arrives at Stoke Manor and begins inspecting Helen Stoner's quarters, he notices the nonfunctioning bell pull. He observes
that it is fastened to a hook just above where the little opening for the ventilator is.
He easily sees that it is used to create an opening in the room—an opening large enough for the snake to enter.