Wow, a lot more added on this during the night. The OP has added that he was told there was no laws that prohibited this, and just not to hurt the grass. So that's not really "permission" per se, but rather, he checked to see if there were any prohibitions. Which there wasn't. And mind you, that doesn't really even take needing to talk to a live person to ask "can I?" either. A person could accomplish the same thing by looking up city and muni codes for himself. But SUFFICE IT TO SAY, he got the green light from the police in that town.
So for who therefore say that he should just "keep on going" and "ignore her" or "wait for cops to show up" or tell her where to sh*ve it, etc... I think this too (while seemingly defensible) is dangerous to his "green light". Let me explain: There has been NO SHORTAGE of people posting stories similar to this, who got challenged in-the-field. You know the drill: a cop or a gardener or a miss-lookie-lou who thinks you're about to make a mess or bother earthworms. So the md'r gleefully whips out their permission, or cites their authority (lack of prohibitions, etc...). As if to think that such a status affords them a carte-blanche to just waltz wherever and whenever they want. While that may be technically true (that no prohibitions exist, or that they have written or oral permission), it is STILL not advisable to keep on in-lieu of a griper. Here's why: In many such situations, guess what happens? The cop, or gardener, or lookie-lou gets on their cell phone, calls down to city hall (to whomever you allege to have spoken to), and says something like : "But he's tearing the place up !". Of course that's not true, but .... guess what happens to your "permission" ? It gets promptly revoked
Thus, no, it's not always a good idea to think that simple lack of specific rule saying "no metal detectors" or simple verbal permission or say-so that there's nothing addressing detectors, therefore means that you can just waltz over beach blankets in the middle of busy-lunch-time traffic.
Thus, you have to use a little frickin discretion. If there are no "rules", or you "have permission", BEST TO KEEP IT THAT WAY by being as little-visible as possible. Why swat hornet's nests guys? Kind of like nose-picking: not necessarily illegal, but for pete's sake, we ALL use a little discretion in our timing, so as not to offend folks, right? So same for detecting in manicured turf: Pick low traffic times. Not high noon when the gardeners are working that particular park (EVEN THOUGH IT'S "OK" or "legal"). Not high noon when miss lookie lou is out front watering her flowers.
It's gotten to where I do most of my turf hunting at dusk, or early early Sat or holiday AM's, etc... So peaceful. So serene. Now for beaches, relicky sites, etc.... cow pastures or fallow fields type sites? Not the same connotations, so wouldn't apply.