It may be that someone is travelling in that coverage area so the radio registers there and you hear the traffic? I'm just grasping at straws here.

You can do what I do: set the scanner up on whatever site you want to monitor and set a wildcard to pick up every TG that shows up on the site. I usually let it run for a week or so. I then look those TG up on the RR database and see what they are. Time consuming, but it builds a list of the actual traffic on the site. There's probably better ways to do it.