hogman, why didn't you just go, if you had proof-in-hand of no prohibitions? As far as their saying:
" the Forest Ranger over the area can make restrictions ":
I believe this has to do with a legal in-field governance, of any/all public land, for any issue that comes up (ie.: not just detecting, but just generally, "for the public good", blah blah). It works like this: A publically appointed rep. (gardener, cop, ranger, etc..) is given latitude, by his superiors to "interpret as he sees fit", so as to apply laws and rules, to apply to a myriad of situations, as they may develop, in the field. For example: A law may say: "No nudity in the park". And let's say you show up wearing a single sock. If you try to tell the officer: "but officer, I wasn't tecnically nude, I was wearing one sock, and I find no prohibition telling me I can't", that cop is still going to tell you that you were nude. Right? And when/if you appeal to higher authorities above him, they're going to side with him.
You see, I too had such a debate, when I got booted from somewhere, where there was no particular prohibition. And I too thought "there has to be an actual rule, not just an arbitary and capricious opinion of a cop having a bad day". But the more I looked into it, I learned that ... legally ... they are allowed to interpret and make things location specific, if an in-field or on-site authority, thinks they've got something inparticular exception, in their locale, etc....
But getting back to your particular case: I still would have just gone. If someone had an issue, they'd have been welcome to tell you. And then you would be "apprised", so no harm done. But here's the rub: odds are NO ONE would have cared less. You merely precluded yourself from a hunting site! (by making yourself a target for them to need to "address this pressing issue") I mean, unless you were being a nuisance in some other way, like waltzing over people's beach blankets, or crashing an archaeologist's convention, I bet no one would have cared less, or even noticed you. I mean, what were you planning on doing, wearing neon red and yellow, and going in front of every kiosk there you could find, begging for attention? A clear cut case of "sometimes no one cares till you ask" here.