There is no shortage of places (like you're saying for your Canadian area, for instance) that have NO laws that say anything about metal detectors. Ie.: No prohibitions or rules. HOWEVER, that's not to say that someone can't morph something ELSE to apply. Usually the dreaded "alterations" and "defacement" type verbage (d/t the necessary temporary evil of extraction). To which I would say that all such verbage about alterations and such apply to the END result. That is: if you leave no trace of your presence, then presto, you have not alterED or vandalizED anything, now have you? Sure, someone can take issue with that interpretation and semantics. Of course. But for pete's sake, were you planning on wearing a big red "x" on your back, going at high noon, waltzing over beach blankets, begging for attention?
Or another thing someone might morph to apply, is the cultural heritage type verbage ("archaeological stuff") that capt-cook alludes to. But again, for pete's sake, were you planning on parading your historically sensitive stuff you find for all to see and know about? I mean, you're only finding NEW stuff, and just looking for that boyscout ring your dad lost when he was a kid there, right?
Bottom line is, if we all ask long enough and hard enough, of enough deskbound bureaucrats and bored rangers, you can ALWAYS finds yourself a "no", even for the most innocuous of modern sandboxes. There can ALWAYS be something morphed to tell you "no", when someone connects dots in a number of various ways. But the REALITY is, detecting goes on all the time, and 99% of the time, as long as you're not being a nuisance, and snooping around obvious monuments in front of archies, no one cares. Oh sure, there might be a certain "hot spot" (a particular ranger or particular park you're told "scram" from). Just avoid those one spots, or one person. But no, I do not interpret that as "broad rules" or to mean that "the entire city, or state or country" etc... is off-limits.
One time, many years ago, a guy I know was hunting a school yard in a city near me. An after-school janitor spotted him, and came out to boot him. The skittish md'r was getting ready to leave, but asked the janitor: "well how about the school down the street, can I go there?" The janitor .... getting irritated.... says "no, not there either". So the md'r says ... "hmm, ok, then how about the park downtown, can I detect there?". At this point, the janitor was getting even more irritated and just blurts out "you can't do that anywhere in this city, it's not allowed on any city land". So the dejected md'r left with his tail between his legs. He spread the word to other hobbyists in our area that "detecting in such & such city is illegal". And some of those other md'rs took him at his word (afterall, how do you argue with a duly-appointed city official?). Many years after this happened, in about 1979 or '80, some buddies of mine started working a park in that town, and were finding gold old silver. I joined them, and we were all doing pretty good, night after night, whenever we cared to go. But in the back of my mind, I couldn't help but remember, from years earlier, that this town was supposedly "off-limits". But no one ever griped, nothing was ever said (as long as we used obvious tact and discretion in our timing, stayed neat, didn't "stick out", etc...). Now that was 30+ yrs. ago, and to-this-day, you can detect any park or school in that town, and not have a problem. So from this story, you can see, that SIMPLY because one person (even a cityhall person you're asking, or a city person or ranger booting you, etc...) who says "you can't do that here", DOESN'T MEAN MUCH TO ME. So unless I personally see an actual rule that says "no metal detectors", I will detect.
Now SURE, of course use common sense, and give lip service and comply with whomever "has an issue". I mean, sure, if you return 30 minutes later, and the same person sees you, then ... duh ... that's not wise. But on the other hand, someone simply saying "you can't do that", to me, does not constitute law. To me it means: just avoid that one person, or park for awhile.
