Look at this steep flight of stairs and the length of the corridor! That disposes of the incident of the door.

Whoever unlocked it was not the murderer." Merrington retraced his steps along the corridor.

As he walked, his eyes roved restlessly over the tapestry hangings and velvet curtains, and took in the dark nooks and corners which abound in old English country-houses.
"Plenty of places here where a man might hide," he muttered, in a dissatisfied voice.
At the head of the front staircase he paused, and looked over the balusters, as though calculating the distance to the hall beneath.

Then he descended the stairs.
It still wanted half an hour to breakfast time.

There was no sign of anybody stirring downstairs except a fresh-faced maidservant, who was dusting the furniture in the great hall.