Did someone accost you in the field and tell you this ? Or was this something told to you at city hall when you were tallking to them ?
Then if you leave no trace of your presence (I assume you fill your holes, fluff them up, etc...) then .... seems to me you're not violating this.
"Permission" ? Your prior ability to detect there was based on permission ? Yes: that makes it easy for them to "revoke it" now. But if it was never dis-allowed to begin with, why did you need their permission in the first place ? (such that now you're on their radar as something they "permit", and so forth)
How did you get that information ?
You'd be surprised how often "holes" gets thrown out there as the "go-to answer" for the "no" they're about to give you. And the md'r is muttering under his breath
"durned those guys that must've left holes". When, in fact, perhaps there was never an incident (or at least not one that would have led to a rule). All it often is, is simply the knee-jerk mental image that a pencil-pushing desk-jockey has, when tasked with thinking about metal detecting. Doesn't mean there was ever actually a case. It can simply the the "go to" reason to answer your "pressing question" type scenario.