Watching You by Arne Dahl

Someone is watching.

At each abandoned crime scene there's a hidden clue: a tiny metal cog. Someone is sending Detective Sam Berger a message, someone who knows that only he will understand the cryptic trail.

Someone knows.

When another teenage girl disappears without trace, Sam must convince his superiors that they’re dealing with a serial killer. As the police continue the hunt to find the latest victim, Sam is forced to unearth long-buried personal demons. He has no choice if he is to understand the killer's message before time runs out.

Somebody is killing just for him.