If you want to get real technical, I'd bet they'd say "no probing" either. Is anyone really going to go watch you retreive stuff, to really even know dig vs probe, vs whatever? I hunt parks, where it is a given that it is ok (well, at least no one ever bothers us), and we dig holes all the time (Explorer deepie silver at 8 or 9" that you can't simply probe to). So whereas, at these parks, it might be ok to detect, I'm sure they'd tell us the same thing "no digging". But bear in mind that their image is open holes and messes. We re-plug and fluff up the grass, and no one is any wiser. Naturally, we are discreet, and if busy-bodies are watching closely, we make sure not to be in the middle of a hole, at that moment. In your mind, you know you're going to leave the place impecible. But to the person watching, they only see your plug open, and assume you might leave it that way.
And as for "power lines", I can't imagine them being that shallow! Sure, even PG&E company has signs up "Call before you dig!" with an 800# to call. I heard of a guy once who was told (he went to city hall somewhere and asked) that, in order to dig, he had to call PG&E for each signal he intended to dig. When the MD'r objected that he was only going a few inches, not deep enough to reach pipes or wires, the desk-bound bureaucrat looked longer and harder at the codes, and tells the guy, "sorry, it doesn't specify how deep, so therefore we must assume any digging, at any depth" Doh! C'mon, you know you won't hit pipes and wires, so just go, and be discreet. If someone's being a busy body, just cruise around and pick up surface clad for a few minutes, till they get bored and walk away.