Forensics set up floodlights, with one propping the door open, and the photographer began to take shots, the surgical forensics mask covering her nose insufficient to block out the ripe odours permeating the space. She had seen murder scenes before, but not like this. Corstorphine pointed out the crisp packets and the still-functioning hourglass which she duly photographed, leaving the charnel house, relieved to take close-ups of the now disassembled door lock in the fresh air.

“What do you make of it, sir? Could he have been locked in by accident?” Frankie looked around the blood-spattered room, working out how many days he’d been trapped in there. Her eyes avoided the laird’s body – the expression on his face would live with her forever.

Corstorphine merely indicated the hourglass, the papal coin shining in the cold floodlit glare. “Dust the whole place for prints, especially the door lock, crisp packets and this hourglass contraption. Then bag the lot and look for any DNA. This is the same killer.”

He felt an anger building up inside him. Three murders and still nothing to go on. It felt as if the murderer was playing a game with him now, deliberately placing clues to identify himself. Even the mode of death: snare for the gamekeeper, church bell for the minister, imprisonment for the sheriff. There was a certain poetic justice in the way the deaths had been engineered, something almost psychopathic in the attention to detail and cruelty of each.

“Sir?” PC Lamb called him over to the lock, disassembled on a spare white forensic suit on the forest floor. “Thought you should see this.”

Inside the lock new metal parts stood out, shining and contrasting with the original rust-coloured components. A tightly coiled spring had unwound, engaging to push the deadbolt into the locked position. A single bone was jammed against the mechanism, preventing any possibility of unlocking the latch bolt.

“Do you think this could have caused the deadbolt to operate without requiring a key?” Corstorphine directed the question to the locksmith, who was attempting to pack his tool box whilst casting furtive glances into the building.

“Aye, looks like it. May have been the door slamming shut that set it off. Poor bastard didn’t stand a chance of getting out. How long do you think he was in there?”

“Thanks for your help. This is an ongoing investigation and we can’t give out any information. I’d be grateful if you didn’t tell anyone about this in case it harms our enquiry.”

“No, of course. Glad to be of service. I’ll be on my way. Goodnight.”