A sign on private land adjacent to the C+O Canal:
When a landowner wishes to keep his/her property off-limits to others, he/she posts signs along the boundary that indicate that the land is private property and others may not trespass without permission.
In common vernacular, such land is described as being "Posted." For example, a hiker might tell a friend, "I'd like to take you there to see that rock outcrop, but the land is posted" or a hunter would say, "You can't hunt there, it's posted." That descriptor--saying that land is posted--simply means that the owner has put up some sort of sign indicating that people should stay out.
However, somehow along the way, a clear and simple sign saying something unambiguous like "NO TRESPASSING" or "PRIVATE PROPERTY" or "KEEP OUT" has been supplanted by the "POSTED" sign as in my photo above.
In other words, the shorthand description of a property being posted with signs (i.e., off-limits) has morphed into the sign itself.
Whenever I see such sign, I think, well, all that means is that the property has signs on it. And the owner is an English langauge nitwit.