Also, MOSWIN is a P25 system, which as such wouldn't have "unlisted" frequencies. The frequency data is transmitted over the air on the control channel, and as long as you're picking that up, you will get all frequencies.
The BCD996P2 doesn't have service parameters like the newer SDS200, so that wouldn't limit your traffic, but there may be unknown talkgroups in your immediate area that have not been defined. If you aren't in ID Search (press the knob followed by Scan/Search to toggle between ID Scan and ID Search), you will only pick up the talkgroups you have loaded in the scanner, ignoring anything else. Looking at Phelps County on the MOSWIN system, it seems to be a fairly extensive list, so the other issue may be some of the law enforcement tactical channels that are listed may be encrypted, to which no scanner would be able to receive.
The other things to consider, if you're not hearing traffic, some talkgroups may only be active on either the VHF or the 700MHz towers in Rolla, but possibly not both. So it would be best to monitor both towers for maximum coverage.