You're absolutely right: Too many md'rs who "got the boot" by a cop somewhere, simply assume that this must mean "the place is off-limits". This isn't always necessarily true. It could be just as you say, that the cop is merely responding to some busy-body's complaint, and is going to take the easy solution. But odds are, if that same cop had just been passing you by, he'd have probably paid you no mind.
So unless there's an actual rule saying "no metal detectors", I do not take a single booting, as anything indicative. Those poor cops have to satisfy "miss lookie-lou"s call. And ..... let's face it guys, there's no escaping that our hobby has connotations and draws curious stares. So simply avoid miss lookie-lou and anyone who might gripe.
For example, there was a park about 100 miles from me, that I enjoyed hunting whenever going through that part of my state. But one time, after 10 or 15 yrs. of having stopped there un-bothered, I got booted by a lady cop. Now SOME md'rs would tuck their tail and never return. Or they might march down to city hall to "get her over-turned" or "seek clarification". But instead, since I was just leaving anyhow, I gave lip service, and left. 6 months later, when passing through the town, I stopped and gave it a whirl. No problems (of course I used discretion, and wasn't going to be in the middle of retrievals in high traffic). That was 10 yrs. ago, and I've been back 20+ times and never had a problem again. Thus I deduce the cop was merely responding to a caller, or perhaps was just having a bad day that day, or whatever.