You will only receive traffic from talkgroups that the MARCS users have selected to receive on their radios that are affilitated with (connected to) the Lancaster site. If the only talkgroup that is selected on the MARCS radios is the Lancaster post, that is all you will hear.
As troopers from neighboring posts travel in their areas, their radio may affilitate with the Lancaster site and you will hear their traffic. For example, here in Lima Post 2 is not yet on the air, but I will occasionally hear Post 80 traffic.
Mike