The sheet was roughly a foot and a half deep by four feet wide, and upon it were four dark horizontal strips, each running almost the full width of the document. When Jasmine stepped closer, she saw that they were black-and-white aerial photographs, but something about them was very odd. They were almost like negatives, so much blackness dominating the image.

‘They’re an infrared aerial survey of the Glasgow area, taken in overlapping latitudinal strips. We do them every five years. It lets us identify premises that are losing heat, which shows up as white on the photographs. If they’re a customer, we advise them on ways to reduce this and burn less gas.’

‘Is that not a bit turkeys-voting-for-Christmas?’ Ingrams suggested.

McGranahan smiled.

‘Aye, if you were to look at it purely on a raging capitalist running-dog level.’

‘But I suppose this was before “Tell Sid”.’

‘Then or now, waste is never good business.’

‘Did you say it was two surveys Jim requested?’ Ingrams asked.

‘Aye. Four of these sheets per survey. Each one took four or five nights’ flying to complete. There’s minor inconsistencies because of small temperature variations, but you can only see it if you look closely at the overlaps. They do them at the same time of year, for purposes of comparison. Always December. Needs clear nights, so the view isn’t occluded, and cold so that you get a stronger heat contrast. The red-stoppered ones are the first survey, from December 1978, green stoppers are the second, from December 1983.’

Jasmine and Ingrams shared a look. These were the two surveys taken either side of the Ramsays’ disappearance. One five years before, the other four months after.

‘Did Jim happen to say why he was interested in them?’ Jasmine asked. ‘It’s just, he only mentioned to us to pick them up if you called while he was away.’

‘No, he didn’t. I’m curious myself. He phoned last week to ask, first time I’d spoken to him in months. When I asked what he wanted with them, he said it was one of those things he could only tell me about once it was concluded. He did promise he’d be doing that over a few pints, but he was definitely being a bit coy, which only served to pique my more morbid suspicions.’

‘What’s there to be morbid about?’

‘Now, I don’t want to be giving away my dark secrets,’ McGranahan said with a teasing smile, which betrayed that he’d be crushingly disappointed if they weren’t interested in him doing just that. ‘I remember telling Jim about these once, being my rather pitiful offering in response to all the gruesome stories he was able to tell me about his time in the polis.’

He began scanning the strips, his eyes moving left to right. He didn’t point with his finger, but it was clear that he had found whatever he was looking for.