It would have come later in the book, and it involves Antonio Mercer, our detective, remembering that times with the missing girl, Julie, weren't always so good when they were together. The only other flashback we see of them is more idyllic.
Here it is for the first time:
The last night Julie and I were
together–as a couple, I mean–
isn't one I like to think about too
often. We were at a party together.
We went to a lot of parties those
days. That's what kids our age and
from our kind of families did. It
was somebody's birthday, a guy
named Aaron. We had all been
drinking and dancing and at some
point I ended up dancing with a
bunch of different girls and then I
think I was dancing with no one
until I was sweaty and out of
breath. I was too tight and I
needed to leave and so I went to
find Julie. Someone said they had
seen her out at the pool house, but
when I went out there, all the
lights were out. The door was
unlocked, so I headed inside anyway.
I could make out some shapes in the
shadows. Couples were using it as a
make-out room. I went to leave but
as I opened the door to go, the
light from the moon came in through
the crack and I saw Julie's shoes.
Red and white polka-dot pumps. She
was kissing Aaron there in the pool
house. When I asked her why, she
said, "When it's dark like that,
you can tell yourself someone is
anyone you want them to be."