McDave, marcomo has a point
If there was nothing prohibitting detecting, you had the run of the whole town, before asking permission. So all you did was risk a "no", where no one would have cared, or probably even noticed you (unless you were some sort of a nuisance, or leaving a mess, etc....). But in your mind, you did the right thing because they said "yes". But this is the response that anyone in a position of authority will probably have. I mean, think of it: Did you think they were going to say "gee, why are you asking me? you don't need to ask" No. Of course not. The mere fact that you asked them, simply infers that whatever it is you asked about, needs their sanction, to begin with (lest why would you have asked?). And thus, they are happy to confer upon you their kingly "yes" (or no, whatever the case may be).
And I have to chuckle anyhow when I read of someone who gets a "yes" thus believing this confirms that permission was needed, to begin with, anyhow: Because whomever gave you that "yes", simply didn't have the right set of mental images in their mind. For example: were you sure to mention "holes", "digging", "indian artifacts", "ARPA", "treasure" and "hold harmless clauses" when you asked? In other words, I could just as quickly go to those same persons who gave you the "yes", and just as quickly get your yes revoked and turned to a "no". All I'd have to say is: "Gee, is it really fair for McDave to go enriching himself at the city's expense, when there is 'no collecting' rules on the books?" or "Gee, what if an innocent little child trips and falls in one of McDave's holes, and sues the city?" etc....
I still think that asking if you can go detecting, where there are no specific prohibitions, is like asking if you can fly a frisbee, or skip stones on the pond. Sure, you can find someone to tell you "yes, I allow you to skip stones on the pond. Thankyou for asking". But that doesn't prove that you needed to ask.