At least here in southern California “posted no trespassing” signs are quite common.
Why the posted?
POSTED
Goes back to how laws are written. If you don’t want someone to hunt or trespass on your land, you have to ‘post notice’ (per the law’s language) and it must be done in some specific way and some specific number of times. “Land owner must post notice every blah blah blah…”
So, “Posted!” denotes that this is an official posting described in the law and intended to meet the laws specs/counts.
It’s somewhat more involved than that, and goes back to how post/posted was more common usage, but that’s the gist of it.
.
Where I grew up, the signs were “Posted : No Trespassing, Violators will be prosecuted”.
One day we encountered one that said “Posted : Trespassers will be violated”.
We decided not to hunt there…
[Cue the strains of Dueling Banjos…]
There’s a road near where I live which goes into private property. You can see the road go uphill but cannot see much beyond that because of the trees and the bend in the road. This parcel is surrounded by a marsh and a private lake. I’ve been curious to how many families live in that parcel, but a sign says: “No trespassing. Violaters will be shot.”