Low shelf, back layer, hidden away behind slick bright colours. Old hardback, dust jacket gone tatty at the edges. St Thérèse of Lisieux: The Little Flower and the Little Way.

Conway pulled it out, carefully, one fingertip. Dust came with it. Sepia young one in a nun-veil on the front, pudgy-faced, thin lips curved in a smile that could have been shy or sly. The back cover didn’t close right.

I put two fingers on the book, top and bottom, held it steady while Conway eased open the back. The corner of the jacket flap had been folded in, taped to make a triangular pocket. Inside, when Conway gently hooked it open, was a Yale key.

Neither of us touched.

Conway said, like I’d asked, ‘I’m not calling it in yet. We’ve got nothing definitive.’

This was the moment to bring in the cavalry: the full search team scouring the school, the Forensics lads taking prints to match up, the social worker in the corner of every interview. This wasn’t a scrap of card, fifty/fifty chance of a bored teenager playing attention games. This was one girl, probably four, maybe eight, who had had the opportunity to be at the murder scene. This was real.

If Conway rang for the cavalry, she would have to show O’Kelly all the shiny new good stuff that justified him blowing his budget on a case turned cold. And bang, fast enough to make our heads spin, I would be headed home and she would be paired up with someone with years under his belt, O’Gorman or some other hint-dropper who would find a way to put his name on the solve, if there was a solve. Thanks for your help, Detective Moran, see you around next time someone drops a big fat clue into your hand.

I said, ‘We don’t know for sure that this was actually the key to the connecting door.’

‘Exactly. I’ve got a copy of the real thing back at HQ, I can match it against that. Till then, I’m not calling out half the force for the key to someone’s ma’s booze cupboard.’

‘And we’ve only got the text girl’s word on who put it here and when. It might not even have been here last May.’

‘Might not.’ Conway let the pocket drop closed. ‘I wanted to take this place apart, top to bottom. The gaffer said no. Said there was no evidence that anyone inside Kilda’s was involved. What he meant was, all the posh mummies and daddies would have a conniption about some dirty detective going through their little darlings’ undies. So yeah: for all we know, the key wasn’t there to find.’

I said, ‘Why would Joanne’s lot leave it here, all this time? Why not bin it when Chris got killed and people started asking questions?’