I gave her a cup of coffee, fresh from my plunger. ‘There’s something of Caleb’s I need to talk to you about.’

She sat on my sofa, with no more idea than ever how close she was to my hidden cabinet collection. ‘We shouldn’t talk about the case.’

I’d printed each image from Caleb’s sketchbook, enlarged to A3 size. ‘I want to discuss his drawings. These are a clue. Caleb risked his life for a sketchbook like this.’

Jeannie looked through the enlarged images. She was one of the few people who had faith in Caleb, and our friendship was one of my last true relationships. She’d been nothing but kind to me, before and after the fire.

I revealed a couple of Sean the Adventurer stories. She nodded. ‘They’re good.’

She was intrigued when Caleb’s subject matter changed to a view of the Brunton fire. He’d depicted it several times, as if drafting but unsatisfied and trying again, with a palette of reds, yellows and black. ‘He draws his feelings the way you sing them,’ Jeannie surmised. ‘These pictures are variations on a theme, different verses of a song.’

Now and then a figure or two, recognisably human, appeared. Sometimes it was clearly Caleb himself, yellow-clad, on his bicycle. One striking image showed the cape-wearing Sean the Adventurer character, flying.

‘Who is this?’ Jeannie pointed at a white mark I’d taken for the sketchbook paper, showing through in a spot where Caleb didn’t colour with his usual intensity. Jeannie turned another page.

‘Here it is again. And here.’

I studied the dimensions of the white shape a feeling building, that was part excitement and part justification. ‘Is it a person?’

More pages flipped in her fingers, an animation coming to life. ‘These dark figures are people. Or ghosts. The white figure is the same shape.’

I peered closer. Realisation thrilled me. A white smudge appeared in nearly every image. Someone who’d been there from the beginning, in images of the fossicker’s hut site that Caleb obsessively drew. These sketches were a primary source in Caleb’s history, like Roman art. Who else was there? Why had Caleb said nothing? I had to ask him.

Jeannie was ready to leave. ‘I hope I’ve been useful?’

‘Very. Thanks so much, Jeannie.’

‘I’m glad to help. He’s not just your son, Phoebe. He’s my former student. You know teachers never stop caring.’

Stephen took ages to answer his phone. ‘Phoebe. Is this important? I’m in a meeting.’

Why would I care? I described the white figure. ‘I’m emailing a photo. You need to look at it now!’

‘What? Why?’ He kept his voice low.

‘Someone else was at the ignition site. We’ve got proof. Pictures.’

‘Pictures?’ I heard excitement in his voice. ‘Who took them?’

‘Caleb had them.’

‘Caleb was taking photos?’

He was bright enough for law school? ‘Sketches.’