‘Just keep an open mind,’ said Nightingale.

The staff made us don clean suits and masks before letting us near the slab. We weren’t relatives, so they didn’t bother with a discreet cloth to cover the gap between the body’s shoulders and the head. I was so glad I’d skipped the bent that morning.

I guessed William Skirmish had been an unremarkable man when he was alive. Middle-aged, just over average height, his muscle tone was flabby but he wasn’t fat. I found it surprisingly easy to look at the detached head, with its ragged edge of torn skin and muscle instead of a neck. People assume that, as a police officer, your first dead person will be a murder victim, but the truth is it’s usually the result of a car accident. My first had been on day two, when a cycle courier had had his head knocked off by a transit van. After that you don’t exactly get used to it but you do know that it could be a lot worse. I wasn’t exactly enjoying the headless Mr Skirmish, but I had to admit it was less intimidating than I’d imagined it.

Nightingale bent over the body and practically stuck his face into the severed neck. He shook his head and turned to me. ‘Help me turn him over,’ he said.

I didn’t want to touch the body, not even with surgical gloves on, but I couldn’t bottle out now. The body was heavier than I was expecting, cold and inert as it flopped onto its belly. I quickly stepped away but Nightingale beckoned me over.

‘I want you to get your face as close to his neck as possible, close your eyes and tell me what you feel,’ said Nightingale.

I hesitated.

‘I promise it will become clear,’ he said.

The mask and eye protectors helped; there was no chance of me accidentally kissing the dead guy. I did as I was told and closed my eyes. At first there was just the smell of disinfectant, stainless steel and freshly washed skin, but after a few moments I became aware of something else, a scratchy, wiry, panting, wet-nose, wagging sensation.

‘Well?’ asked Nightingale.

‘A dog,’ I said. ‘A little yappy dog.’

Growling, barking, yelling, flashes of cobbles, sticks, laughing – maniacal, high-pitched laughing.

I stood up sharply.

‘Violence and laughter?’ asked Nightingale. I nodded.

‘What was that?’ I asked.

‘The uncanny,’ said Nightingale. ‘It’s like a bright light when you close your eyes, it leaves an afterimage. We call it vestigium.’

‘How do I know I didn’t just imagine it?’ I asked.

‘Experience,’ said Nightingale. ‘You learn to distinguish the difference through experience.’

Thankfully we turned our back on the body and left.

‘I barely felt anything,’ I said, while we were changing. ‘Is it always that weak?’