‘Go on,’ he says, staring straight ahead. Is she going to tell him that her digital investigator is more than just a friend? It wouldn’t bother him. Each to their own. It might explain why Strover’s romance last year with Sean, the pub conspiracy theorist, didn’t last long.

‘Our own custody database, Europol and Interpol resources, they’re all useful, but there are some much bigger databases out there,’ she says tentatively. ‘Legal and illegal ones, on the Dark Web.’

‘And your friend accessed them?’

Strover nods. Silas drives on in silence. Technology has changed policing so much in the past few years, marginalising old-school detectives like him. He might not be much good with computers, but he still needs to understand what they can do.

‘Faces scraped from every social media site you’ve ever heard of,’ Strover continues. ‘And from a lot you haven’t. Weibo and Youku in China, VK and Odnoklassniki in Russia. Age-filter apps like FaceApp are also generating huge numbers of faces as everyone shares images of what they’ll look like when they’re ninety.’

‘I’d rather not know, thanks,’ Silas says.

‘The same’s happening with Zao, the Chinese deepfake app that lets you swap your face for a film star’s. Combine all these uploaded faces with databases used by the world’s law-enforcement and intelligence agencies, and you’ve got a big dataset,’ Strover continues. ‘A lot of mugshots. More than a billion of them.’

The revenge of the selfie. He’s never understood the urge to ruin a good photo with a gurning face in the foreground.

‘How do you search that many faces?’ he asks.

‘Clever algorithms, fuzzy logic, probability theory. Russian programmers are leading the way. There’s a search site called SearchFace.ru that can sift through half a billion faces on VK and find a match in seconds. But there’s still a long way to go.’

Silas thinks back to the last time Strover’s digital investigator friend helped them. And he thought Strover was computer savvy. This woman was extraordinary, blinding him with talk of symmetric-key block ciphers and triple data encryption algorithms. He’d had to look it all up afterwards.

‘I’m amazed your friend’s only found seven doubles for Rob.’

‘We don’t know exactly who or where they are yet,’ Strover says, reading from her phone. ‘One was last seen in Ireland eight years ago. Two could be in Australia, another possibly in Thailand – the best likeness. The other three were last seen on MySpace and Bebo in North America more than ten years ago.’

The traffic is beginning to move when another call comes in on the intercom for Silas. It’s the Devon and Cornwall Police Control Room again.

‘We’ve located the driver of your car,’ the man says.

Silas knows at once that it’s bad news. ‘Located’ is never a good word.