‘The male we’re looking for has been in the Edinburgh area for years. He was old enough to be walking the streets at night five years ago, so we can narrow the age range down to starting in the mid-twenties. He knows the city well. Advocate’s Close is a back alley that runs beneath the structure of other buildings. Unless you knew it, you might assume that you couldn’t exit at the other end. I’d say something about him gave her the creeps. Women who live on city streets develop good instincts for danger real fast.’

‘Playing devil’s advocate for a moment, what’s to say he wasn’t simply in the wrong place at the wrong time?’

Connie leaned forward, beaming. ‘I like you, Brodie Baarda,’ she said. ‘You ask the questions that make me look good. Why wasn’t he in the wrong place at the wrong time? If I was in Vegas and taking a bet, I’d call it like this. The saliva on the note is the killer detail. We pass DNA every time our skin cells touch something. We leave invisible smears when we don’t wash our hands after using the restroom.’

‘Oh, God,’ Baarda said faintly.

‘No, no, go with it, there’s a point. Blood DNA has more innocent explanations than you’d think. But the loss of saliva is often specific to emotion and trauma. On cutlery, it’s a no-brainer. On the end of a pen, I’d expect it. In a bathroom, loads of it after transference from a toothbrush or the sink. On a banknote? Disgusting, dirty things. No adult in their right mind puts their mouth on it. So the note caught some of his spittle.’

‘He was upset …’ Baarda said.

‘Damn right he was upset. You can add angry, accusatory, and overwrought to that list of emotions. People rarely lose saliva when they’re talking normally. The only other time is when people are fist-fighting or kissing. Do you know that if I kiss you right now, I’ll transfer around eighty million germs from my mouth to yours?’

‘Please don’t,’ Baarda said.

‘Killjoy. Anyway, those are my thoughts, but I do have one question.’

‘Is it about saliva?’

‘No. However, I have many more exciting facts and statistics on the subject of bodily fluids if we’re ever stuck on overnight surveillance together. My question is this. There’s no assault, no body. So how come the twenty-pound note was DNA tested at all? You’re not telling me Police Scotland has such limitless resources that it checks every homeless person’s abandoned belongings based only on a call saying there’s been a disturbance?’