Federal systems can be different from anything else you might listen to. As an example, the Los Padres National Forest has multiple mountain top repeaters. There is only one input frequency and one output frequency. (Actually there are two separate nets) Which repeater you want use is determined by the input PL tone. All the outputs have the same PL. The repeaters are not linked. If you want to listen to the dispatcher and you cannot hear the repeater, you can listen on the input which comes from a mountain near the headquarters. The dispatchers have microwave links to all mountain tops so they can hear the traffic coming back to them. They also have a backup link system in the 406 Mhz area.
Thus, the system you are listening to may have a common input frequency and multiple non-linked sites. The thing to listen for is if you can hear the input, but not the repeater. If there are distant repeater sites that you cannot hear, you will still hear the input. Naturally, there will be no mobiles that you can hear on the input unless they are close.
The use of linking repeaters by federal systems in the 406-420Mhz band was more common 30-40 years ago. The USFS had four separate ones at one time in this area and the FBI had one that relayed radio traffic from Los Angeles into this area. No encryption then either.
