Still think it's best to ask. Once it's clearly defined, there is no chance at all of detecting. The more you push for clarification, the more likely you will get it, and likely not be pleased with the verbage, or signage. Maybe things are different in the big cities, but the city workers are friendly, helpful, not evil. Guess maybe good paying jobs, with benefits are hard to find in town, so they appreciate having them more. I drive 40 minutes to get to work. Doubt there would be an issue, long as you aren't causing problems, creating a nuisance, or people complaining. Like hunting the soccer field, a group shows up to play, you refuse to leave, since you were there first, and you have every right to be there... Even though you are getting some very nice targets, the right thing to do, would be to move off the field, and let the kids play soccer.
Got a hunch there are a lot of places, of questionable historic significance, which cities submitted to the Federal Government, for some of that easy to get grant money. Once they accepted the check, they agreed to certain restrictions and guidelines. The rules about no digging, no removing, alteration, and so forth. Parks are probably pretty easy to get a historic designation, and fairly safe to accept the grant money, since they are seldom slated to be removed, or radically changed.
Why is it so wrong to accept that some places just aren't open to detecting, it's a big world, lots of land to hunt. Are we so blinded by lust and greed, that we need to find some angle, some way to slip in there and get the goods, until caught, asked to leave. Do you change machines, appearance, and go back in a week or two, stubbornly set on getting what you 'rights', entitle you to?
The vague rules and ordinances give us some room, to work with people in charge of a particular site. They have a little room in how they choose to interpret those rules. Talking to people, finding places to hunt, where it's not going to be a problem, ensures that it never gets to the point where it needs to be clearly defined. Asking a city worker, might get you a 'No' at a few sites, but if the city has to update the rules, it most likely will involve most of the city, so they don't have to keep revisiting the same business. You keep taunting the big dog, the big dog will bite.
There are a lot of people who fly under the radar, work the system, avoid paying taxes. How many people actually go to city hall, pay for permits to repair their house, add plumbing or electrical? Usually, if they have to pay a professional, he's got a license to protect. The do it yourself guy, probably not, unless he's looking to get tax credits, break on insurance. Many people get away with it, most of the time. Some get caught, and usually they would have been better off playing by the rules. They really weren't doing any harm, just trying to save a few bucks, avoid a few hassles.
We aren't they only creatures digging holes, and not everyone hunting is going to do a pretty job of filling them in. When you talk to someone, you get a chance to explain what, and how you do it. You are less likely to catch the blame for a mess you didn't create. 'Wasn't me', or 'It was like that, when I got here', while you stand there with your metal detector, and digging tool, probably won't work any better, than when you were a child. Least you won't suffer corporal punishment...