CAD is the closest thing policing has to a flight recorder. It logs when a call came in, when an event was created, which units were assigned, when each changed status, and how the event was closed — all automatically, all timestamped.
Why the search parameters decide the answer
CAD data can be queried by address, event number, phone number, unit, or date range. A search run against only one field can return zero rows while responsive records sit in the same system under a different key. That is why a records response should state what was searched, not just what was found.
The documented example
A five-category FOIA to Chicago OEMC produced a single address-search report returning zero rows. See the response analysis.