I bet you that there is wording about "defacement" or "destruction" or "altering" or "vandalism" or something. But notice in each of those cases, the IMPLICIT effect is, the END result. Ie.: if you leave no trace of your presense, then you haven't "vandalized" or "destructed" anything, now have you?
But you know as well as I, that we will always loose that debate with a busy-body, because of the temporary evil process of extracting our finds. No matter how much we sugar coat it, or in fact truly do leave no trace of our presense: it can and will just turn into a semantics battle over whether or not your plug will turn brown, or whether or not you packed it enough, etc.... And let's face it: even if it's true that you DO leave no trace, put yourself in their shoes: the next "yahoo" that wants to detect, yet perhaps isn't as neat as you, can till say to the person objecting: Goldmanjace is allowed to hunt here, so why can't I? And really now, think of it: do you think gardeners, cops, etc.... want to get into that battle of how long the length of a probe or trowel is, who stomped their plug more firmly back into place verses the other guy, etc...
So it's hard to blame them.
Here's what I'd do: avoid busy bodies like that. If you gripe and try to get yourself sanctioned, fight it, etc... you may find yourself getting a rule WRITTEN, to "address your pressing issue". Why not just avoid that one lookie-lou? Can't you figure out what shift he works that park, and just go at other times? Hunt 5 to 7am on a Saturday, or after dusk, etc..... As much as I'd love to say there's a way to get everyone to "love" us, I'm afraid if someone thinks they can and should be able to look forward to that kind of reception, they've probably picked the wrong hobby. Because let's face it: we're in an odd hobby and draws the stares of the curious, and has implications and assumptions that *could* be made.