Diggem, if someone is worried that there may be something similar in
their areas, to the VERY rare cases you cite, they should look it up themselves in city code. Like, I would assume that city codes/laws are available on-line at city websites, right? Do a key-word search on it for "metal detecting" or "metal detectors", etc.... If city law is silent on the issue, then I would take that as there being no prohibitions in city schools and parks. For example: in the cases you cite, if indeed there is a permit (or prohibitions or whatever), then I would assume that it is in existing written code, that would be found in the existing texts. But if it is silent on the issue, I would look no further for sanction. If you absolutely feel you MUST talk to a live person, phrase it in terms of "is there anything that addresses or prohibits the use of metal detectors on city property?". See the difference? The latter puts the burden on them to show an actual citation.
Because the minute you go couching your question in terms of "permission" or "can I do such & such", you merely give someone the freedom to say "no,
just because I
said so" (because maybe they think you're a moron who'll leave holes), when in fact 1) no real rule existed and 2) no one would probably have ever paid you any mind.
And yes, I'm of the group that you allude to, that feel like "as long as it is public land, and as long as it's not posted, why seek permission". I can't understand why anyone would think any differently, but that's just me. I mean, when you come to an intersection while driving, and see no signs prohibiting U-turns, you assume you can make a U-turn, right? Of course I would not apply this "public" logic to obvious historical monuments, the White House lawn, etc... I mean, c'mon, we're talking city school yards in Billy-Bob town USA