‘I’m thinking: result. Punters’ll be getting let back in any minute. Instead, next thing I see is mair polis and mair dugs headin’ intae the station, like the big search has only just started.’

Catherine recalled the sight of the sniffer dog, snuffling eagerly at that other locker, even able to open the door because the lock had been knackered. She thought of the witness accounts of Callahan and Fleeting on Wednesday night and Thursday morning, two men with a job to do. There had indeed been a shipment waiting for them at Central Station, but it had been removed. Then a few hours later, so were they. For ever.

‘I was with Cairns when he got the tip,’ she told Abercorn. ‘For reasons I can’t disclose, I know for a fact that the call didn’t come from Tommy Miller.’

‘He had a second source? You think Cairns got played?’

‘No, I think I did. By Cairns. I got a message he had information for me regarding the McDiarmid murder: wanted me to meet at a café two minutes from Central, first thing Thursday morning. It was so that I’d be on the spot, buy into it. I even phoned Scotrail for him, gave the order to shut down the station. The call he got was for show, for my benefit; or it was to let Cairns know everything was in place. Either way, it must have come from one of the guys Liam here saw.’

Something else fell into place as she spoke this aloud. The call had definitely come from a cop. He had picked up when Laura dialled the number, but not when Catherine had tried dialling it this morning. On the later occasions, she was calling from her mobile, but Laura had called it from her desk. All outgoing calls from the station showed up as the same switchboard number, so the source must have thought it was Cairns phoning him.

‘Which brings us to the purpose of my visit,’ said Abercorn, reaching into his jacket. He produced an A5-size envelope and pulled from it a stack of photographs. Catherine recognised most of them as cops, with a few villains and a few nobodies thrown in for control.

Whitaker screwed up his face as he pored over them, looking anxious and frustrated. He dallied over one or two of them, but was reluctant to commit. Catherine caught Abercorn’s gaze as Whitaker came to the head shot of Cairns’ big buddy Fletch, confirming their mutual suspicion that he had to be the top candidate, but Whitaker failed to pick him out.

‘I told you,’ he said to Abercorn. ‘I only caught a glimpse. A couple of these guys look vaguely familiar, but that’s all I could say.’

‘What about CCTV?’ Catherine suggested. ‘The station’s full of cameras.’

Abercorn gave her a sour look, like she was missing something obvious.