Re.: "obligatory" signs. Consider the following that happened to me:
There was a particular vacant lot in a small historic little burg. For years it was just vacant, with a short-cut trail across it. No signs, no fences, etc.... And we found a few old coins in the lot. Every other year or so, whenever passing through that town, I'd give it a whirl.
One time though, I noticed it was all plowed and tilled. Ahhh, perfect time to hit it, eh ? Might bring oldies to the surface eh ? So we hit it again. Found a few more coins
Eventually, someone had planted corn, melons, etc.... in this small lot. And ... like all the years prior, we still went. Still no signs, still no fence, still never anyone around, etc.... Each seasonal plow had the chance to reveal another coin or whatever.
One time however, I arrived to find a uggghhh, a "no trespassing" sign.
But since we were already there, I decided it would be ok to check the dirt road along the side. Figuring that *technically* we would not be "in" the lot.
So as we were going along, flirting with a nebulous boundary, .... out of the corner of my eye, I saw a truck pull up to the curb. A man got out and was dealing with gardening equipment in his truck. It was clear that he must be the person tending this garden. Ooops !
So we slinked further back to the borders. And .... slinked back to our vehicle parked near his. He saw us, yet never paid us any mind. We put away our gear, and went somewhere else to finish our detecting day.
A few weeks later, I related this story to one of my employees who lives in that town. He busted up laughing because it turns out, that older man was his DAD !
He explained that his dad had permission to grow veggies on that lot. Which was owned by some sort of developer or something. So I asked my employee :
"How come for a year or so, it was a garden, with no sign. Then a sign all of the sudden went up ?"
He said the ONLY reason his dad had put up a sign, was the following reason: A week or so prior to that, he had arrived to see someone stealing vegetables. He jotted down the license plate # of the truck, and called the sheriff. By the time the sheriff arrived, the veggie thief had already left. When the gardener showed the sheriff the lic. place #. But the sheriff said he couldn't do anything about it. WHY ?
"Because the lot is not posted". Doh!
Hence the gardener posted the lot. My employee said his dad could care less if someone were walking on the dirt road, or md'ing, or whatever. He just didn't want anyone stealing his veggies.
No do not construe this to an extreme where I'm advocating going in people yards, etc.... Just using this as an example of what the OP could be saying is the meaning of "obligatory". There are scores of signs that are put up so that if some numbskull slipped on a banana peel, he can't sue the school district, or sue the railroad Co (did you know RR tracks are private property?), and so forth.
The trouble with this question, is there are a million shades of gray. The minute someone suggests a spot is "innocuous", then someone else will envision md'rs helping themselves to front yards of obviously private homes, etc.....