The empty safe probably is the first object that suggested to him the grim tableau of robbery and murder, which he arranges in order to ensure his own safety.
"But remember one thing: no miscreant was seen to enter or leave the house surreptitiously; the murderer left no signs of entrance, and none of exit.

An armed burglar would have left some trace--_some one_ would have heard _something_.

Then who locked and unlocked Lady Donaldson's door that night while she herself lay dead?
"Some one in the house, I tell you--some one who left no trace--some one against whom there could be no suspicion--some one who killed without apparently the slightest premeditation, and without the slightest motive.

Think of it--I know I am right--and then tell me if I have at all enlisted your sympathies in the author of the Edinburgh Mystery." He was gone.

Polly looked again at the photo of David Graham.