He stepped noiselessly inside, and closed it noiselessly behind him.

He was in a narrow hall, where a few yards away, a light shone down a stairway at right angles to the hall itself.
"Rear door of pawnshop opens into hall, and exactly opposite very short flight of stairs leading directly to doorway of Isaac's den above.
Ramshackle old place, low ceilings.

Isaac, when sitting in his den, can look down, and, by means of a transom over the rear door of the shop, see the customers as they enter from the street, while he also keeps an eye on his assistant.

Latter always locks up and leaves promptly at six o'clock--" Jimmie Dale was subconsciously repeating to himself snatches from the Tocsin's letter, which, as subconsciously in reading, he had memorised almost word for word.
And now voices reached him--one, excited, nervous, as though the speaker were labouring under mental strain that bordered closely on the hysterical; the other, curiously mingling a querulousness with an attempt to pacify, but dominantly contemptuous, sneering, cold.
Jimmie Dale moved along the hall--very slowly--without a sound--testing each step before he threw his body weight from one leg to the other.

He reached the foot of the stairs.