It was the thing that every retrospective failure in the case – every oversight – would always come back to: if there had been upturned furniture, or damage to the walls, or blood, if it had looked in any way like someone had been hurt here, then the houses would have been pulled apart, top to bottom. But it didn’t. It looked like four households had cleaned their properties, locked up and then left. And maybe they had, or maybe they hadn’t, but it was why the bugs – leaving aside the fact that they were so well hidden – were never found when investigators arrived here. It was eventually why the case lost momentum in the media as well: because as compelling a mystery as Black Gale was, as much as the papers would have loved to have continued running stories on it, there had never been any evidence of wrongdoing. People had the legal right to disappear – the legal right never to be found too – and that was a hard reality to swallow when you were looking for a headline to sell.

‘Maybe the bugs weren’t even here two and a half years ago,’ Healy said, his thoughts echoing ones I’d had myself. ‘Maybe they were installed afterwards.’ But then he stopped and we looked at each other, the same question in both our faces. Why install a bug in an empty house? It was of value now, when someone was actually looking into what had happened, but – apart from a few true-crime bloggers, and amateur sleuths like Tori Gibbs – no one had been actively looking into Black Gale since the media lost interest and the police investigation hit a wall. It was much more likely that the bugs were in here before any of the villagers left – and that, in turn, changed the entire perspective of the case.

It made it something else.

Something worse.

I thought about the hard drive in the cavity wall, about the devices dotted around the house, and about the way the technology monitored and reacted to our sounds. The soft hum I’d heard, the sound creeping out from under the sofa that had helped me zero in on the first bug, didn’t seem to start until I’d strayed within range.

Before that, the device had been silent.

‘I think the bugs have got motion sensors in them.’

Healy frowned. ‘How do you figure that?’

‘It would explain why Mills arrived here when he did. These devices go into hibernation when there’s no movement in the house, so when we returned here this morning and began looking around, we set them off again. And as soon as we did, they sent out confirmation that there was fresh activity in the village. Some kind of pulse.’

‘A pulse?’