There should be a command to end patrols for specific vehicles. This way if you have a patrol set for six hours but need to move around personnel or move vehicles between stations etc you can recall them to the station.
Another thought would be being able to mark a vehicle out of service (status 6) while out of the station so that they can return and not be recommended as part of response plans. (This is already done when their "shift ends" for vehicles with automatic out of service times and they are on a call. They return to the station while in Status 6.)