And another one.
This doesn't apply to your typical police procedural. Those ones are all about the police characters investigating the crimes. And are usually really boring.
This applies to a thriller in which the detectives/police are not the central characters, and much of the action is happening to another main character. That main character is telling us, the reader, everything that is going on - what happened in the past, why it's happening, what might happen in the future. They're usually aware of some/most of the events surrounding the murder/crime and their relation to it. We primarily follow them as they try to solve it, or find a way out of their predicament.
However, we'll also get the odd chapter from the police officer/detective investigating the same murder/crime. They'll find clues, and they'll share theories with co-workers about what they think has happened.
The problem is, the reader already knows. The main character has already told us. But we still get pages and pages of the secondary cops On The Case chasing around clues and delivering theories. I suppose it's to get us to understand how they arrive at the right conclusion and help to save the day, but it's basically just repetition to bolster the word count.
This happens ALL THE TIME in Lisa Gardner's books. She may be one of my favourite writers, but sheesh! The books are always divided between a main (usually) female character caught up in a terrible situation, and the (usually) female detective investigating the terrible situation. In the past, the detective has usually been D.D. Warren. She is always five steps behind the other main character in figuring out what is going on. Why do I need D.D. Warren telling me what she thinks is going on, when the main character has already told me what is actually going on?
Lisa Gardner's lastest - "Crash And Burn" - thankfully managed to subvert this trope by having the main character not even really know who she was. So we, the reader, were as much in the dark as everybody else was. Unfortunately, it was one of her weaker efforts. The first 40% was literally just an investigation into a motor vehicle accident. A bit too much Research On The Page there.