Wow, sorry to hear that...
Yep, never ask for permission unless a sign expresses you need it, or of course where detecting is not allowed due to historical nature of the site or so on. Only seek permission from higher ups when detecting there requires a permit or it has been expressed that detecting is not allowed.
These government types just love to say no to anything in life if it doesn't follow their own personal outlooks on how us minions should be led in the way they feel. And, many times they'll say no to something just to cover their butt due to any potential lawsuits should somebody get hurt in various activities that they are on the record as saying "go ahead". We can thank the lawyers and sue happy people for that aspect of the ball of wax as well.
Only when one's back is against the wall and they have no other choice should they seek up an official approval. To do otherwise is just asking for an official "no", where then you can no longer claim ignorance to such things.
That's why if I do ever run into some busy body do-gooder who can't stand to see people having fun, and they start complaining to me about what I'm up to, then I'll just say something like "sorry I upset you" and leave to come back another day. Chances are I'll never seen them again there.
I figure if I try to start throwing my rights around with them they'll just call the police, and then guess who gets kicked out just so they can grease some squeeky wheel Joe Citizen so they don't have to take another drive out there in the same day.
I have a few friends in the police, and one thing they hate is to have to answer to their supervisor why a situation didn't get resolved the first time the call was made. That's why when you see these cops shows even if their was no violence in a domestic dispute, they'll strongly "encourage" one party to leave for the night, so they don't have to explain why they had to show up again later when things hit the fan again.
We may feel we have the right as tax payers to detect any public park, but the truth is we are far too few in numbers, and namely far too few in money to sway policies our way. Just not enough votes behind us to make them fall in favor on our side of things, so the best thing to do is just stay under the radar as much as possible and not be too much in the public's face with our activities. I try to stay away from people and always keep my back to the nearest road or person when I bend down to dig. Less seen the better IMO...
