‘Everyone in here talks as if they’re one and the same. But you don’t think so.’

I shrugged. ‘It comes down to this. If Jack the Ripper was around today, would he Tweet? Would the Boston Strangler be updating his single status on Facebook? I don’t think so.’

Wren laughed. ‘You’re dead wrong. I think that’s exactly what they’d be doing. Are you kidding? Jack the Ripper would have loved social media. The Boston Strangler, the Yorkshire Ripper – they would have got a big bang out of the digital community. Taunting the law, puffing out their little chests, revelling in the horror, the horror. The digital world is made for sociopaths. As long as you don’t get caught.’

‘Then you don’t think we’ve done enough to find Bob.’

She pushed a strand of red hair from her face. ‘Clearly not, because we haven’t found him, have we? But it’s not necessarily DI Gane’s fault, and I’m not being diplomatic. Bob’s hiding behind multiple firewalls, and IDS – intrusion detection systems, like a burglar alarm for computers – and Tor, the onion router, where every message is encrypted and re-encrypted multiple times through countless servers. So it will probably take more than a love letter to flush him out.’

Wren pushed back her chair and picked up what looked like a thick exercise book – the Action Book, which logs the tasks that have been done and the tasks that are yet to be done, and who should do them. It was the real-world equivalent of the Action Management page on HOLMES. Mallory still liked to have a paper record of everything.

‘I see you’re going back to Potter’s Field today with a Specialist Search Unit, and taking more statements from the boys who were out running with Philips,’ Wren observed.

I nodded.

As well as monitoring the Action Book, Wren was responsible for all the statements coming in, logging them into HOLMES, and for maintaining the integrity of the chain of evidence, the paper trail leading from the crime scenes to MIR-1 and all the way to court.

It was a big job, and I could see that it bored the hell out of her.

I indicated the laptop.

‘Is it true what they say?’ I asked. ‘Does everyone leave a digital footprint?’

‘Now I wouldn’t go that far,’ she said. ‘Everybody leaves a digital shadow. We all live two lives – our physical life, and our digital life. All of us. And the finest minds of my generation are currently working out how to sell you stuff you’ve already bought. So there’s pixel tracking, page tagging, tracking codes – it’s why all these ads seem to follow you around, to magically know it’s you.’ She laughed. ‘At its hollow heart, the internet doesn’t really want anyone to be anonymous. Because it wants to sell you stuff.’

‘So everyone leaves a shadow.’

‘Yes.’