Charles, no, it's not un-safe to assume, after having read the laws/regulations. Why would someone be "assuming", if they've looked things up, and found nothing that says "no metal detecting"? You can simply do a key-word search under variants of "metal detecting" or "metal detectors", etc.... If nothing is there, then nothing is there. Presto. How is that "assuming"?
But I know where you're heading with this: the "assuming" part you're referring to, is that someone STILL might not morph something ELSE to apply, right? Ie.: the dreated "alteration" or "defacement" type clauses (digging). If that's what you meant, then give it up now. Find another hobby. You will NEVER get permission to "dig" "deface" and "vandalize" any park. I know some people have gotten permission to "metal detect" in their parks. But notice how they probably used euphamisms (and probably didn't use words like "dig" and "holes" to begin with). And if they DID, it means they didn't ask far enough up the ladder of chain of command. There are ample stories of persons who got a "yes" from city hall, only to be accosted by an angry gardener or cop. The md'r proudly whips out his "permission" to show to the griper. The gardener or cop simply gets on his cell phone, calls down to city hall, and says something like "well I don't like it. He's digging up the park, blah blah ", and your permission is promptly revoked! So you see, either way (whether with permission, or without), you and I STILL have to .... uh .... be a little discreet, no matter what. Eg.: if you see them mowing/tending this park that day, choose another park. Go at low traffic times. If an archie convention is going on there that day, probably not a good day to go. If you see someone studying your every move, don't be in the middle of deep retrievals when that busy-body is watching, and so forth.
Please be aware that: YES, anyone in authority can interpret/morph rules all day long, to apply to things as they see fit. Ie.: you're right, there doesn't NEED to be a "specific prohibition" saying "no metal detecting". Laws in all arenas are purposefully written vaguely, so as to apply to a myriad of circumstances that may arise in the field. And authorities are given a wide degree of latitude to interpet, as they see fit, lest there be constant arguing of semantics. So for example, you have broad clauses like the forbiddance of things that are "annoyances" type clauses. So with something vague like that, a cop can stop you from singing the star spangled banner at the top of your lungs to little kids on the swing set. You can try to say : "but there's no rule forbidding the singing of the star spangled banner", but you know he'll morph something else to apply to you, right? So you're right: they can morph the dreaded digging and alteration type clauses to apply, at any time to us. This STILL doesn't mean that we should therefore all go down to city halls and seek permission though. Because to do so, is ALSO a quick way to get "no's", when ..... a lot of times .... no one would ever have cared less (till you asked). And as I say, even if you DID get a "yes", even that is always subject to over-ride, still subject to bootings, etc....
So in this hobby you simply have to face the fact that if all this bothers you (that you might have someone take issue with you, and you might have to "move on" or "give lip service" now and then), then you might have to stick to private property or sandboxes.