I too had the same question several others here did: In your OP, you said yourself about the park, that you: ".... know is absolutely legal to detect ( my friends sister is a lawyer n detects there ).... ". Yet you turn around and feel the need to ask someone "can I?" anyhow? As the "polite thing to do" ? Why can't the "polite thing to do", be to follow the rules, that you already know about?
As your case exactly shows (and so many are starting to realize): This psychology has manifested itself once again, and your example clearly shows it: Sometimes no one cares .... UNTIL you ask.
I mean, for example, I bet that lady would never have noticed you, nor given the matter thought, till you came in with your "pressing question". Persons in authority tend to take the "safe" answer. And when you think of it, ... the moment anyone comes in asking "can I do such and such" simply infers that something must be askew with the activity. Ie.: harmful, damaging, evil, etc... that you had to ask permission, to begin with. Lest why would you be asking, if it were innocuous and harmless? Doh! This is not subconsciously lost on the person you are asking, and their mind will drift to all sorts of things to morph to fit the question, in order to tell you "no". When ... truth-be-told, perhaps they'd never have given the matter a moment's thought, and never have paid you a second glance.
The sad thing now, is that when that SAME ranger sees ANOTHER md'r in that park, guess what's going to to go through that lady's mind? She'll remember this earlier inquiry, and think: "aha!, there's one of THEM", and start booting others. I've seen this happen multiple times. And thus ... another law or policy is born
So in the future, as folks here are telling you, look it up for yourself. If you see nothing that says "no metal detectors", then presto, it must not be prohibited.
Re.: the psychology of a pretty girl verses an ugly guy asking. I suppose in cases where "permission was needed" (which isn't the case in this case), then that *should* help, I would think. Heck, my wife gets out of tickets all the time when driving, and when it comes my turn, I don't get that sort of leniency/warnings, durnit!