Twisted one, you say:
"
but no government entity has full listing of the entire realm of legalities on their website. "
Yes, I agree. It is simply impossible for any list of laws to list everything that is illegal, to solve every possible problem of semantics. Therefore laws are written in vague enough way, so as to apply to a myriad of circumstances as they may arise in the field. For example, there may be a law about nudity, and someone can try to rationalize that by wearing a single sock, they were not technically "nude", and so forth.
But let's cut to the chase about your comment, as it applies specifically to our hobby: You are concerned that even though there may not be an actual specific rule forbidding metal detecting, yet you might run afoul of something
related, that some busy body interprets, on the spot, to apply. Right? For example "defacement and alterations" or "no collecting" vebage, or "cultural heritage" verbage or "lost and found laws" verbage, and so forth, right?
Sure, it's entirely possible that someone could come up to you and morph a TON of things to apply to you. If this is your concern, then I'm afraid you've chosen the wrong hobby (or your simply going to have to restrict yourself to private land). Because to start down that road, is to have lost the battle already. Because I gaurantee you, that if you want to fear things that cops might morph to apply to you, then give it up right now. For example, the dreaded "alteration" type clauses: I challenge you to go into any city in the USA, and walk in to city hall, and ask "
Hi. Can I please alter and deface the park?" and see what they say.

Of COURSE they are going to say no. But as you can see by a quick look at Tnet's various finds forums, people are detecting parks all the time, because they choose
not to define themselves in that light. And so too do passerbys care less as well, apparently, as evidenced by the fact that there is no shortage of park hunters, eh? But sure: if you want to worry about the
potential that someone may not love you or your hobby, then you've chosen the wrong hobby. Because let's face it: we're in an odd hobby, that has connotations, and draws the stares of the curious folk out there sometimes.
If in the isolated incidents that some authority DID morph something to apply to you, and you DID point out to them that you looked up things first, and saw no prohibitions, then they are welcome to "inform you" of something they have *now* intrepretted to apply to you. No problem. You just pay them lip service, and avoid that one person or place in the future.
As much as I'd love for parks to have signs that say "metal detecting welcome here, go ahead, come dig up things for your personal profit and enjoyment", it just aint gonna happen. And I can always find some rule to forbid me, if I do enough antics and worrying. So ..... therefore .... I will only consider myself forbidden in the case of SPECIFIC wording saying so, or a warning telling me so. Otherwise, you and I might as well give up the hobby now.