Gimmie, you say that sometimes you "cannot find any info" on whether certain parks are "legal to MD there". I assume this means you checked the park's rules (usually available at the city website, or on the wooden sign at the entrance, or the brochure they pass out at the pay-booth, etc...), and found nothing addressing the issue, right? Then if it is silent on the issue, I have never heard of any "arrests" type stuff. If it is not specifically prohibited, wouldn't that be like getting a ticket for making a U-turn where there was nothing prohibiting U-turns?
But here's the rub: Naturally ANYONE can be a busy-body. There are other rules someone could morph to apply to you. Like "no collecting" for example, which is normally a rule in every park, everywhere, so as to prohibit people from backing up their pickup truck and helping themselves to the sod, flowers, sand harvesting, etc.... for personal gain. Or the dreaded "digging" or "defacement" type clauses. Heck, you might do nothing more than jab a screw-driver probe for surface clad, and some busy-body could try to say you're "destructing" things.
It's odd how arrests, confiscations, jail, etc.... get bandied about as things to fear, and reasons to assume we need to cower and ask permission where detecting is not specifically dis-allowed. Yet when you ask those people for examples of any such arrests, it grows strangely silent. If there are any "arrest" or "ticket" stories, it's usally someone sneaking obvious historic monuments, posted sites, or someone who couldn't take a warning. If there are any examples of someone getting hassled legally for detecting innocuous non-posted, silent-on-issue parks, I would say that's the EXTREME exception. I mean: so too do you occasionally hear about some motorist being roughed up by an over-zealous cop, for nothing but a tail-light out, right?
I'm about as brazen as they come. When I travel to other cities, unless there's something right on the sign at the entrance, I assume I'm ok, until told otherwise. I've had this attitude for 30+ yrs. and never had any tickets, arrests, problems, etc... Sure now and then you get a busy-body. So what?
I also generally hunt at off-low-traffic times, so as to avoid busy-bodies, to begin with. Like, if you see that's park maintenance day, pick another park that day, etc.... Because let's face it: we're in an odd-hobby that "sticks out". When people see the guy "swinging that geiger-counter thing", they ask themselves "just what is he doing?" "is he going to hurt the grass?" "is that allowed?" etc... So if you go at off-times (early AM's, after 5pm, etc...) you will not make yourself an object of attention, to begin with.