Fair answers AARC. You say:
....There are NO "laws/rules" to "look up" as you say in MOST instances here.
Not like its posted somewhere online.. OR at a park on some sign ...
Hard to believe that there are zero rules of use. Oh sure, perhaps not on the park sign. But ANY forum of public land (city, county, state, fed), HAS to have laws/rules/ordinances somewhere. If you don't know where that is, you can always take this approach: Ask them: Where can I avail myself of the laws (city charter, muni codes, etc...) ? If they say "what is it that you wanted to know?", you stick to your guns and say : "To see the laws , park rules, etc...". They will direct you to a net-link, or to where it exists in binder form at the counter at city hall, etc...
And even if it were true that there were "no laws", then gee, to me that simply means that there is no law prohibiting me to metal detect, eh ?
.... So... that being said... You have to ask to know for sure...
Curious : If there is "no laws", then what would they be basing their answer on ? Personal Whim? Mood? If I had the the feeling I'd just been given an arbitrary safe "no", I would not at all like that. I might be tempted to ask "what is that based on, and where is that written?"
.... when I have asked and been told yes .....
And on the surface , those "yes's" would seem to substantiate that "it's a good thing I asked", right ? Actually, even a "no" could cause you to conclude the same thing: " gee it's a good thing I asked, otherwise I could have been arrested". Hence whether a "yes" or a "no", either answer seems further proof that asking was a necessary step. Right ? The MERE FACT that they can answer "yes" or "no" subconsciously cements that notion. Otherwise, how could they have answered either way, if there permission hadn't been necessary from the git-go ?
But authority doesn't work like that. For example, can you imagine them answering in this way: "That's a funny question. Why are you asking me? You don't need my/our permission, if it's not prohibited". No. Authority never does that. They will bestow on you their princely "yes" or "no", since, afterall, you asked. And that merely implies their-say-so was needed (lest why else would you be asking?).
So a "yes" someone gets (or even a "no"), does not mean to me that permission is necessary. And also various persons have gotten "yes's" which they whip out to show any busy-body. But the busy body gets on their cell-phone, calls to city hall and says: "But he's tearing the place up!" (which isn't true, of course). Then guess what happens to your permission ?
Yes a "yes" is better than a "no", don't get me wrong. But I'd just be worried that I could likewise have gotten a "no", where, quite frankly, detecting isn't an issue, nor ever has been.