A 302 documents an interview, and is only a part of the larger case file.
(from Q link)
In conducting investigations, employees of the Federal Bureau of Investigation (FBI) are required to record all activity and document all information using case files. The case file is the central system for holding these records and managing investigative resources. As a result, the case file includes documentation from the inception of a case to its conclusion.
The FBI uses the case file to manage all criminal and intelligence gathering activities, as well as for personnel (support) and administrative matters.
Q seems to be saying an entire case file is missing. Which logs will point to it? That's the mystery that must be solved.
