How about when they give you permission to Metal Detect, and then when they see you dig a hole, they go balistic. That happened to me at a KOA in Utah a few months ago.
I don't let them see me digging If I suspect someone is watching me (like the person who just gave me permission), I will purposefully dig a few surface targets (you know, the obvious zinc penny signal that is barely below the grass, that I can find with just a poke of my screwdriver). If you do this for 3 or 4 targets, the lookie-lou will invariably grow bored, and return to their business.
You gotta have good peripheral vision to sense when you're being watched. Or, for the first 10 minutes, if you were in a position of just getting permission, and know that that person is on this site somewhere, assume he may be watching/curious, and purposefully dig some shallow/surface stuff.