Now....this is just my opinion, but you're making the same mistake over and over. You're asking permission.
OK, I say this somewhat with tongue in cheek. But to be honest, unless I’m hunting in an area where it would be blatantly wrong to hunt without permission (e.g., my neighbors front yard, the lawn in front of the police department, etc.), I don’t ask. Kinda the old ax, “shoot first and ask questions later.” I’m just very respectful when I get kicked off a hunting ground, and try to explain how digging holes in the ground is a good thing (aerates the grass, allows more water to get to roots, if done correctly no one should be able to tell that I was there, i.e., filling holes and replacing plugs, etc….believe it or not, that actually worked once…once…yeah, it surprised me too).
The main reason people, companies, businesses, construction sites, don’t want you on their property is because of “liability”. At least this is what nine out of ten property owners will tell you. I don’t think one out of ten people actually knows how far liability goes in the event of an accident, but no one wants to take a chance.
However, if you don’t ask permission, generally people (particularly the city) really don’t care if you metal detect. And you’re doing them a favor, because they are not liable if you are trespassing (ho-ho).
Another very good tack is to stop at the local police station and ask them to look at the “books” and see if there is a local ordinance that restricts MDing. Ninety-nine percent of the cities in the U.S. have no such ordinance (let’s hope it stays that way…it probably will because MDing is not a rampant sport that is taking over the world). Then, at least, if you are hunting on public property and want to tactfully justify your actions while being kicked off the property you can say, “I did check first with the Police Department to make sure there were no ordinances against MDing here in town.” This line works really well if the person kicking you off the property IS a policeman. Even if you hear that ugly word “trespassing”, he’ll most likely let you off with a warning.
There is one place where you never want to tread! That is National Parks. MDing is forbidden in all National Parks (they are national) and the standard operating procedure is to issue you a ticket and take away your metal detector, which you may or may not see again (getting it back will be a big hassle at the very least). However, if you handle the situation right, and you’re lucky, you may even get off here with a simple warning. The trick is to be very good natured and friendly when accosted by a NP policeman or ranger, and to ask a lot of questions so that they can feel that they have educated you. I once was hunting on a beach which, honestly, I had no idea was part of the National Park system surrounding the San Francisco Bay Area. And I really didn’t even realize how much trouble I was in when approached by an actual San Francisco Police Officer. I was just my normal friendly self, joking around and finding out from the police officer what the limits of the national park were, when he said, “OK. I could write you a ticket and take your detector away, but I’ll just ask you to leave.” Hey, it pays to be friendly.