To add to what inigo88 said, because of the nature of the trunking system(s) that SD county uses (RCS for county; SD City has its own dedicated TRS) not all of those zones and channels will be receivable in every area. If the stream you're hearing is located in the north county, for example, that receiver may not be able to scan the RCS South Zone well, if at all. Therefor, even if the scanner in use has all of those zones and channels programmed in they won't necessarily be active - it depends on whether some of those users roam within the coverage area of the North Zone trunking controller site and whether those users even have permission to operate on that Zone (I think all fire units have full system-wide permission, however). A user with a primary usage in the South Zone, for example, will only routinely be heard on that zone and may or may not be heard on the other zones depending on what talk group is being used; some talk groups are dedicated to the local areas while others may have county-wide coverage. Also, if, for example, a user normally operating in the South Zone travels up north that user may have roaming privileges which allow him/her to stay on their normal talk group and still communicate with their fellow group users down south (it's all patched through using the "smarts" in the trunking and site controllers). In that particular case, if a scanner user has that talk group programmed in to their North Zone talk group list then they will also get a chance to hear that "southern" traffic, but only so long as that south user (or other south users) remain in the North Zone area - once they leave and go back home it will go dead again. It works in reverse for the South Zone, of course, as well (and the East and Northeast zones).
Put simply, it's a "trunking thing". Most users program their scanners according to where they are located in SD County in terms of what site/zone they are closest to as well as what they are interested in. In general, it doesn't make sense to program all of the county services, police, fire, etc., in one huge RCS file as as that eats up memory space needlessly unless they really need to monitor those normally out-of-zone talk groups for any roaming users. The stream you are listening to probably covers SD city and maybe the South Zone of the RCS so it that scanner may not have the other zones programmed in or be able to hear those other zones anyway.
I'm one of those "anal" listeners that put a lot of out-of-area talk groups in my scanner programming especially in the case of the fire stuff because they will tend to roam a lot especially during peak fire season when a lot of mutual aid is required. I do routinely pick up Monte Vista CalFire traffic on the North Zone as well as Heartland and El Cajon CHP - I would imagine that they roam close enough frequently to the North Zone coverage area on a regular basis to allow me to do this and/or are commonly patched across zones.
More than you wanted to know, I'm sure, sorry - got a bit carried away.
BTW - Orange County is a completely separate system - it is not part of the SD County RCS.
-Mike
