All of this is listed on our federal parks, local parks, and city ordinances....
Huh ? Just curious : What city is it, that you say you've found the specific "No md'ing" rule. I'd be curious to search their muni-codes for myself, and see what you're referring to.
If it's true, that every single speck of land (fed, state, county, and city) ALL have "no md'ing prohibitions", then .... I have a sneaking suspicion of how that came to be. But first, I'd like to look into this myself. What city and what county are you in ?
And you're not referring to ambiguous catch-all verbiage like "harvest and remove", or "alter and deface" are you ?
.... I do have a question for you. If public land doesn’t have something displayed/listed you will detect it? ....
You are asking a 2-part question. Because when you say "Displayed", I'm assuming you mean: The wooden sign at the park. Right ? Eg.: Dogs on leash, closes at sunset, etc... Right ?
But when you say "Listed", I'm assuming you're referring to something that is perhaps on the books down at city hall, yet not necessarily on the wooden sign at the park entrance ?
To answer your questions: When I go city to city (like when traveling) I do not hesitate to hit any park I come to. Unless there were a sign saying "no md'ing". And assuming, of course, that it's not an obvious historic sensitive monument.
But you are absolutely right, that perhaps it's not on the sign, yet *could* conceivably be in the minutia at city hall, right ? Personally I don't fret myself to that degree. But if a person is skittish, they can check out city muni park codes for themselves. In this digital age we live in, most every city has their charter, their muni codes, their park rules, etc.... somewhere available for viewing. Like on their website somewhere.
If it's a super small city or county that simply never put their charter, codes, laws, rules, etc.... on-line, then you can accomplish it this way: When you go to city hall, you DON'T ask "Hi, can I metal detect ?". Lest you become the latest victim of
"No one cared till you asked" safe-answer routine. Instead you say: Where can I find a list of all the municipal laws, rules for park -usage, etc.... Because it MUST exist in print-form somewhere. Eg.: In binder form down at city hall or the library. Or they'll give you the direct link. And then once you have the comprehensive list, if it doesn't mention md'ing (ie.: is silent on the subject), then presto: Not disallowed.