The piece of paper had no title, and its edges were covered in doodles characteristic of Aaron’s often wayward brain. But as Hawkins scanned the printed information, she felt the familiar spark of connection.

She’d heard Aaron mention the information two days ago, and had reminded him at the time to add it to the investigation board and the case file. But it had only been a moment ago, as she read through Colin Wilson’s recent journey-planning history, that she had realized this hadn’t been done.

Now she held a list of the last fifty routes programmed into the sat nav in William Tennent’s BMW.

‘Grab that phone off my desk,’ she instructed.

‘Yes, ma’am.’ He disappeared back into her office while Hawkins scanned the list, returning seconds later holding the handset.

‘Bring up that navigation search history.’

‘Right.’ He faffed for a moment before looking up. ‘Got it.’

She began reading out the place names in reverse order, both start and end locations, most recent first, waiting each time for Chris to look for a match, but he took so long on the first two that she switched them round, letting the trainee read his list in order, checking each destination against her own.

Fifteen comparisons in, still without a match, Hawkins’ optimism was starting to fade. They were already going back over a month, reducing the chances of any matches being more than coincidence. Hits were likely to be something generic, an amusement park or the O2. Her attention began to wander, brain already building contingency plans should this lead fail.

But she glanced up at Chris when he said the next name. ‘Did you say Thundridge?’

He checked his screen. ‘Yes.’

‘Hertfordshire.’

‘Yes.’

‘It’s here,’ she said. ‘When did he last select the route?’

Chris fussed with the controls. ‘Six weeks ago, ma’am. Late October.’

‘Spot on. Wilson did his search then, too.’ Hawkins pulled out her own mobile, began tapping the same location into its route planner.

The trainee watched her, an edge of excitement entering his voice. ‘But this place is nowhere near Brixton, ma’am. What do you think it means?’

‘I don’t know.’ She selected the most direct, forty-minute drive to the small village in relative countryside north of London. ‘But you and I are going there right now to find out.’

21

Hawkins slowed the Alfa, taking in the sweep of Hertfordshire countryside spread out before them. The road’s tree-lined margins had given way to houses. They were nearing their destination. She checked the time: quarter to four. The winter sun was approaching the horizon, signalling the imminent onset of dusk.