You'll hear talkgroups whenever there's someone on that talkgroup that's using the site you're listening to. A site will broadcast all talkgroups that are active on that site and nothing on the system that doesn't have active users on that site. This is to reduce the number of channels necessary for the system to operate. Think of how many frequencies each site would need if all 10,000+ talkgroups all over the state were broadcast on every site in the state. Lots, that's for sure. To keep things managable, only traffic a site carries is for users that are actually using that site. The rest of the talkgroups are ignored on that site since it's broadcast on site(s) where there are actual users on that talkgroup.
For example, a PD unit from Tickfaw is up in Bossier City picking up a prisoner and his radio is still on his local talkgroup. You'll hear the traffic on that channel if you're on the Bossier City site. When he starts driving south again and moves to another site, you'll stop hearing that talkgroup.
