One thing you can bet on when talking to any bureaucrat or administrative personnel, if you ask for any permission you will not get it w/o a tit-for-tat. Encountered city engineers while hunting in the park where work was ongoing over the years. In this town 2 out of 3 that approached were curious and showed no negative response and were positively influenced by my interaction with them. One approached and was mildly abusive but I informed him on the history of public service in America and his role as a public servant, not a public master. He relented when I offered to let him carry off the broken glass, nails, aluminum shards and mish-mash in my junk pouch.
On another occassion the kid and I were being filmed by a park district official during a carnie cleanup and I approached him in a friendly manner and the more kindness I showed him, the more power he assumed until, blaming me for all the divots in the ground, even the holes made by the carinval rides and attractions, he crossed my line. So I let him have it. We were surface hunting, not digging anything. But I threw (figuratively speaking) five pounds of everything from the shredded cans to the SHARP-PONTED curtain hangers (that carnies leave behind in the hundreds) at him, along with the drug -use items including syringes, crack pipes from the carnies and gave him the public service message. He relented, apologized and the nearby local P.D. watched in amusement.
Moral of the story, don't fight city hall, wait until they come out their rat holes and face off with them in public, watch them scurry off like the roaches they are. DO NOT get timid with public servants unless you enjoy the concept of living in some People's Republic where only approved activities are allowed.
All this said, if it says no trespassing or is behind a fence, respect that, also recognize that city hall is a very local thing where people worry about local attitudes, make sure your's is bigger than those who want to deny you. The Feds however are a very different story, STAY AWAY!!! The State also takes much more understanding on our part and you must become familiar with those laws that affect us and respect those you cannot change or work around.
While I enjoy my self-described status as a militant metal detectorist, I won't give anyone an excuse to nail me. It just don't make sense to take something fun and make it criminal. Always be prepared to demonstrate the positive side of your hobby! I have a five gallon bucket in the trunk that gets emptied when full of the broken glass, razor blades, screws, nails and other dangerous items that I routinely recover, rather than attempt to discrimnate out. Those items you discrimante out may just serve a higher purpose in the end than that one great coin or piece of jewelry you find. Especially when those are things you most commonly share with your local elected reprehensibles!
Good Luck and Happy Hunting!