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Cowell shrugged, which Poe took as a no.

‘Show him the next document, please, Tilly,’ Poe said.

Bradshaw placed a single sheet of A4 on the table. It was a printer test-page. The kind used to isolate problems between computer and printer. It was all square and circles and lorem ipsum, the dummy text used on documents and brochures when the real text isn’t yet available. Parts of the page were in colour and parts were in black and white. Small font, big font, small circles, big circles – the entire page was filled. It was stained, in an evidence bag and, up until that morning, had been in Robert Cowell’s bin.

It didn’t take long for Bradshaw to point out what was obvious to them all: the imperfections on the test-page stood out like they’d been highlighted. There were more of them as there was more ink on the page, but the imperfections she’d identified on the documents wrapping the Secret Santa mug were all present.

‘They match, don’t they?’ Poe said.

Cowell looked at him in confusion.

‘I’ll recap for everyone. The documents used to wrap the mug that contained Rebecca Pridmore’s fingers match the document we found in your bin, the kite you say was stolen was found in a tree overlooking her back garden and when the police raided your house this morning your sister ran and then told you to say nothing.’

Poe turned to the solicitor.

‘Do you think a jury will find that circumstantial, Mr Lear?’

Lear said nothing.

‘Tilly, please show Robert the documents recovered from the next two crime scenes.’

Bradshaw laid out blown-up segments from the A4 pages found at the church and Fiskin’s Food Hall.

‘This was found on Christmas Day at a crime scene in Barrow,’ Poe said, pointing to the document from the church. He then pointed at the one found in Fiskin’s Food Hall. ‘And this was found on Boxing Day at a crime scene in Whitehaven.’

Bradshaw indicated how the flaws on the photoreceptor drum had left corresponding marks on the two documents.

‘You can see that a different photoreceptor drum was used on both these documents,’ she said. ‘They do have blemishes but they match neither those on the printer test page that Poe found in your bin nor the ones that wrapped the Secret Santa mug.’

‘So why show us them?’ Lear said.

‘I’d pay attention,’ Poe said, ‘because you ain’t gonna believe this.’

Chapter 40

‘When Miss Bradshaw matched the flaws on the document found in your client’s wheelie bin to the flaws found on the paper used to wrap the Secret Santa mug, she was using what’s ca

lled a passive printer identification technique,’ Poe said. ‘It means the marks are unintentional. A by-product we’re happy to exploit. But, and I could scarcely believe this when she told me, there’s another printer identification technique and this one isn’t passive, it’s active. It’s called yellow dot tracking and it rendered redundant all the steps Robert had taken to make it look like the three murders were the work of three separate killers.’

‘What the hell is yellow dot tracking?’ Lear said.

‘I’m glad you asked,’ Poe said. ‘Tilly?’

Bradshaw turned over seven photographs and laid them out in a row.

‘These images are heavily magnified sections of each of the seven A4 pages connected to this case,’ she said. She pointed at the first four. ‘These are from the pages used to wrap the Secret Santa mug.’ She pointed at the next two. ‘These are from the documents found at the church and the food hall. The last one is the document Poe found in your wheelie bin.’

Cowell and Lear both leaned in.

Lear looked up first, his face showing the same level of confusion Poe’s had when Bradshaw had explained it to him earlier.

‘What are we supposed to be looking at?’ he said.

‘Yellow dot tracking is an active technique where traceable data is explicitly and covertly embedded within the body of a document,’ Bradshaw said. ‘The dots are arranged in grids and are printed in a regularly repeating pattern across the whole page. Each grid can encode up to fourteen seven-bit bytes of tracking data. They are embedded into each document about twenty billionths of a second before printing commences. They can only be seen under blue light or under a microscope.’

‘You’re kidding?’ Lear said. ‘What about privacy laws?’

‘It’s been around since the 1980s, Jon Lear. No laws are being broken and the public don’t have a right to be informed. Most of the major manufacturers use active printer identification techniques.’