Even without considering the wildlife, it's heavily wooded hillside, with plenty of briars and brush mixed in.
I try to not go on the hillside without boots that have thick soles. I end up digging Osage thorns out of the soles of the boots. I've also got quite a few of those wonderful trees that have long thorns growing out of the trunks (can't remember the name of the tree). Then there's the briars, poison ivy, steep slopes in some areas (including the area near the house they are renting)...
Judging by where I initially heard the voices coming from, they were well over 100 yards across the property line and definitely out of sight of their house.
I had been planning on getting new signs and putting them up before deer season. I guess I know what I'll be doing tomorrow afternoon.
ETA: I called my sister and told her about it. She sold me the parcel that adjoins the rental property, a couple years ago, and she had some trouble with a previous tenant of that rental property thinking that the hillside was part of the rental property and completely fine for him to move fences, remove topsoil, dump trash...
Her first comment was "IT'S NOT A PARK!!!".