I use Discovery periodically on several local systems that are adding users and talkgroups. The Ellis County layer of
FWRRS initially only had Midlothian & Red Oak. But last summer, Ellis County itself, as well as the county seat (Waxahachie) agreed to join, along with upgrading the layer to P25 Phase II only and adding three additional sites. Running Discovery, I've been able to identify new talkgroups for the two new members, and have started to see talkgroups coming into play for another city in the county, Ennis, which also disclosed plans to join. There is at least one more city that will join (Palmer), but I have not seen a projected schedule of when that will occur.
You can set Discovery to only log & record new talkgroups, or any activity. It will also collect recordings of activity for each talkgroup, that you can playback & use to establish the agency & usage. If you 'find' new TGIDs, but they are service types not of interest (such as utilities or public works), you can add those to your programmed system & set to avoid to thin out the herd that Discovery finds.