This sentence is slightly confusing because the word "case" has several meanings here. The first, implied meaning is that the file regarding bribery is part of a "case", an ongoing investigation.
The case number is on her computer.
Here the author is talking about a "case number", a code that lets the police identify a specific investigation. This code is on her computer, so we know that Carolyn was aware of the case, and most likely working it.
But the case itself isn't among the papers in her files.
Here we are talking about a physical object, something that might be found among papers or files. This is the physical case file, a folder containing all of the forms and collected evidence that make up the investigation.
Since Rozart had been describing the case in abstract terms when discussing the investigation and the case number. Rozart describes "the case itself" to make it clear that he is specifying something in particular, in this case the physical object.
For comparison, consider a collector who buys a car at auction. The collector pays money to become the new owner of the car, and they will be provided with paperwork that confirms they are the owner. However, the car will need to be delivered, so it may be some time before they take possession of the car itself. We have gone from discussing ownership in the abstract, to ownership in a specific and particular way.