Guys in suits exploring woods?

I wasn’t sure what to give this for flair. My brother owns some property and has been seeing guys walking around in his woods in black suits, unmarked black suburbans without plates are parked on the road that connects to his property. He has called this into the DNR and police who said they would look into it but no response from them. I’m just curious if anyone on here has seen anything similar to this or maybe has an idea what might be going on. He owns over 100 acres of land