Andy had reached the point where he was going to keep going until someone told him he could stop. Even in the bright morning sunshine he looked grey and tired. The next search was staging at Bircher Common, where there was enough room for police and volunteers to park. I took Andy Marstowe aside behind a Peugeot Van with battenberg visibility strips and a West Midlands Police crest, and asked him whether he knew anything about Nicole Lacey’s invisible friend. He just stared at me blankly and said he didn’t know what I was talking about. I’d have preferred it if he’d demanded to know why I was wasting his time. Which just shows, you should never wish for things you don’t really want to get.

‘What the fuck is this bullshit?’

Derek Lacey stared at me after I asked him the same thing. He was red-faced and erratic and, if I was any judge, about a day away from coming apart at the seams. His voice was angry but his eyes were sad, pleading, wanting to know why I was tormenting him with these stupid questions. I got him calmed down using the patented reasonable police voice while making sure I stayed out of reach. Fortunately, it’s easier to settle people in plain clothes, the uniform has a tendency to set people off, but either way the important thing is to remain calm but firm. This is where doing your two-year probation in the West End comes in really handy.

I explained that we, and it’s always ‘we’ when dealing with aggravated members of the public, were double checking every possible point of contact between Hannah and Nicole and the outside world.

‘When kids talk about imaginary friends,’ I said, ‘sometimes they’re talking about a real person. You see, say you don’t want a child’s parents to know you’re talking to them . . . so you tell the child not to tell anyone, tell them that bad things will happen if they do. But kids like to talk, they especially like to talk about their friends. Especially if they’re interesting or naughty. I mean, what is the point of interesting or naughty if you can’t talk about it to someone else?’

A strange look came into Derek’s eyes and I wondered whether maybe I should have avoided the whole ‘stranger danger’ aspect of my little speech. Served me right for making this stuff up as I went along. Then he pushed his hand through his thinning hair and took a deep breath.

‘Yes,’ he said. ‘I see now – apologies. What was the question?’

I repeated the question and he shrugged.