From what I understand, state law is that private property does not have to be posted to still be private property. Some guys I know got in trouble with that this deer season, and tried to say that it shouldn't matter because the land wasn't posted. In the end, they got let go on it, because there was no obvious boundary, like a fence, tree line, ditch, what have you, but they've been warned. It's not like there's not a ton of public and corporate land available to hunt on.
You're supposed to know where you're hunting, and if you hunt private property, you need to have permission. Seems obvious to me, but I'm a property owner who gets trespassed on almost every year, in addition to having the signs that I do put up torn down and vandalized.
I have no sympathy whatsoever for the kind of person who would trespass onto someone's private, residential land and shoot an elk there.