Did the children feel it too? If so, they showed no sign. Instead, Julia took up a position of almost military significance: a visual crossroads that took in both directions of the track, the house and its grounds, and the forest – as far as could be seen. Victor began counting paces from the treeline in. The boy stopped at fifteen, just as he was lost from sight.

‘Victor?’ he said, entering the forest himself.

‘Here, Inspector.’ The boy waved from between two trees. ‘My studies have shown that fifteen paces is, on average, the distance required to become fully hidden in a forest.’

He stared at the boy, at a loss for words.

‘Of course, that is a gross simplification. Variables such as the season, deciduous as opposed to evergreen forest, the ocular capability of the subject… I have extensive charts, should you wish to learn more.’

‘Another night, perhaps. Why are we here, Victor?’

‘Why? To observe, of course. Which requires an observation post.’ Victor walked from tree to tree, looking back to the house when he came to each one. He bobbed and ducked his head, birdlike, only to move on again in as much of a straight line as the forest allowed. From time to time he looked up, just another motion in this odd dance? No, Christophor eventually understood: the boy was checking the branches, those that had large enough and low enough branches to be useful as an ‘observation post’.

If my night-brother found the situation even faintly ridiculous, he gave it no thought. He had encountered characterful individuals of all kinds in his occupation, but few with the drive for scientific endeavour and logic of this pre-pubescent boy that he now followed through the forest.

‘Aha! Mark this tree, Inspector.’ Victor stood waiting; he meant it literally.

Christophor carried a small knife. He wouldn’t have said it was for such eventualities, but in truth the marking of a tree was closer to its usual employment than anything more martial. I had seen it used to pry open jewellery boxes, pick up objects without touching them, and scratch all manner of surfaces. Now, it was used to scratch VM – denoting once more the Virgin Mary – into the bark of an unsuspecting tree. Not just a mark, but a mark against witches. Victor approved.

‘Why here, Victor?’

‘I have already explained, to observe.’

‘Yes, but what?’

‘Well, what can you see?’ the boy said.

I did not expect such patience from Christophor, but he looked about him. The forest offered little, yet he could make out a surprising amount of the Eder house. The observation post was directly in line with the plane of the front of the building. The side was obscured in places, but mostly that was the higher windows and the roof. And of the rear it had an almost complete view of the moonlit greenhouse doors.

The doors.

He glanced down at the boy.

‘You’re watching the entrance and exits of your home.’