‘Surveillance at her house didn’t report anything.’

‘I know. He’s being very careful.’

‘He might have switched cars, the old Deepak Tripathi trick,’ Gillard said. ‘Maybe Morag drove the car here.’

‘Maybe. In any case, the car hasn’t moved in five hours and forty-five minutes. We’re keeping it under surveillance in case he comes back for it.’

‘He’d be stupid to do so.’

‘Yes, but he must be running short of options,’ Townsend said. ‘We’ve got his mobile and his wallet, so he is on a pretty short piece of string. I assumed he would try to buy a burner phone, so Tracy here ran a proximity path analysis to see if we could find it. And we have.’

‘That’s very impressive.’ Gillard said, coming around to look at the screen.

The young woman pointed at her screen to an immensely complex tangle of threads of constantly changing rainbow colours. ‘It’s a piece of software that sits on top of the switch, or what used to be called the telephone exchange. It searches the movements of millions of mobile phones in real time, looking for parallels in the cell towers triggered, and turns it into a map.’ She homed in on the huge knot, gradually magnifying one part of the screen. ‘This parallel track is quite indicative.’

‘So one of these is the satnav flagging its location?’ Gillard asked.

‘Yes, that’s the base path, the one we know for certain,’ Tracy said, pointing at the screen and further magnifying it. ‘And this track is so close that we can only assume it is from the same vehicle, or perhaps one travelling in convoy. It triggers all the same cell towers at the same time.’

‘What do the colours mean?’ Gillard asked.

‘Ah. This is a probability map, adjusted for route frequency. So, for example, a packed commuter train heading for Waterloo will have hundreds of correlations on the same bits of network many dozens of times a day, so it will be light green, indicating a low probability of significance. Then if, say, six of those commuters get on a bus outside the station, we may move up to dark green or orange. It’s really when the pattern is not only identical in space and time, but on a low frequency path, that the software allocates it high significance.’

‘So if two phone traces travel on a minor road at the same speed and time there is a high probability they are together in the same vehicle.’

‘That’s it, sir, you’ve got it.’

‘So you identified the phone. Was it still in the vehicle when it was parked in the multi-storey?’

Tracy checked the screen and brought up a background map. ‘Yes. Having identified it, we followed the burner phone moving at pedestrian speed for a further ten minutes before it was switched off. However it was on a street which should have CCTV.’

‘That is fantastic work,’ Gillard said. ‘Get me the details, and I’ll get the team onto it.’