Just to slightly add to the above, occasionally, on the Ellis County layer, I would hear Mansfield PD units on their TGIDs, as well as occasionally ambulance to hospital talkgroups for other layers. Mansfield is one of the only three sites on FWRRS that is not simulcast (the others being SW Johnson County, and the TMS site). In all likelihood, those units are getting so far south that they can't access their home layer, and end up, for a few minutes, on Ellis County's layer.
I added the Mansfield TGIDs, as well as the EMS/Hospital talkgroups, to my Ellis County system, with those departments set as avoid. That way, they do not keep popping up. I keep the Ellis County system set on 'ID Search', looking for new talkgroups as Waxahachie, Ellis County transition more users to the system. At some point, we'll also see Ennis & Palmer start popping up, as both have committed to make the move.