"Guard Duty" is an assignment other than a regular radio call, such as, guarding a prisoner at a hospital, a crime scene, road closure, etc. The assignments are based on citywide coverage and availability and come from HQ, also referred to as "580". The car could be assigned from any zone/precinct in the city.
If you have questions about Columbus or the feed you are also welcome to use the "Contact Broadcaster" function. Many questions are also covered under the Feed Notes (I will add his question to that area). Thanks For Listening.