... I wonder if nothing is on the books and they ask you to leave and you refuse , what happens
Ah, excellent legal question. Here's the scoop: There is nothing to stop a "duly appointed public official" from making a judgement call that you are afoul of some law (even if not "
specific"). And then yes, their superiors will usually side with them, if it came up for appeals, etc... . Because you see Dwight, some laws are PURPOSEFULLY written vaguely, so as to as to apply to a myriad of circumstances that may arise in the field. In other words, it's impossible to truly list every-single-thing that is "illegal". That's why there's vague laws that .... for example .... forbid "annoyances" or "disturbances", and so forth. Then officer (or any public worker) can do his job, when odd-ball things arise. I mean, if it WASN'T that way, then society would all into chaos, and people would be forever debating cops about semantics, in the field. There HAS to be leeway for a cop to do his job, because various nuances of issues can arise.
Yes it's arbitrary. Yes it's capricious. Yes it can be "whimsical". But barring something glaringly un-fair, most of the time, when it comes to routine things like "
whether or not something was harming the grass", then .... truth-be-told ..... most superiors down at city hall, or judges, or whatever, are going to side with the rank-&-file person in the field. Because if they didn't (if they perpetually sided with the John-Q-public), then as I say, cops could never get their jobs done, and would effectively have no power to do their job, and their job morale would plummit.
But the mere fact that something *could* be morphed to apply to my hobby (the usual damage or alterations type verbage) does NOT mean that .... therefore, I need to, or should, go down and "head it off" at the pass, and ask "can I?" type questions. I mean, so too could a frisbee "poke someone's eye out" or "annoy" someone, etc... But no, even though that *might* happen, no one thinks they need to ask "can I fly frisbees". Sure we can debate that one is more inherently prone to produce damage to the other. We can debate how accurate or analogous that comparison is. Sure they're not identical in associated risks or complaints. But it does serve a point. And if you START with the ASSUMPTION that your hobby is inherently dangerous, wrong, evil, damaging, stealing, avoiding taxes, and breaks harvesting and collecting clauses (if all those things are "automatically" equivalent to detecting), then I have to ask that person: Why the heck did you ever get into detecting? I mean, didn't it occur to them that they might have to "Dig" to find the target?

Or that not all archies are going to love them?
Thus yes, to answer your question, if told to "move on", then sure, give lip service. (and then avoid "just that one person" or "just that one park" in the future for awhile). But no, it doesn't mean that therefore, we need to ask.