Good question supersonic. Because anytime questions of detecting public land comes up (even innocuous places with no rules prohibiting!), a few are sure to post fears we're supposed to leery of: "Getting arrested" "tickets" "confiscations" "jail", and so forth. Yet when you press those people of examples of such things, few are ever forthcoming. And if any examples ever ARE given, they are invariably someone night-sneaking an obvious historic monument, or someone who couldn't take a warning, etc...
I'm as BRAZEN as they come, and have been so for 35+ yrs. of this. And I have never faced anything more than scrams.
But with that said, I do personally know of 2 people who got tickets: One was for state historic park property, AND HE WASN'T EVEN DETECTING. He had been detecting a private ag/furroughed field, adjacent to that park. And then when done, he took a short-cut across a portion of the state park (just a dirt/grass over-flow parking area for RV's), on this way back to the public street. Detector off, just walking to his car. A ranger saw him and gave him a ticket. He could have fought it (since he was only going from point a to point b), but when he saw that the ticket was only a measily $100 or so, he just paid it. Wasn't even worth taking time off work, and he actually got a laugh out of it.
The only other time was a friend at a federal military base. It was admittedly historic-in-nature. The two buddies each got tickets. And of such a insignificant amount, that it was more of a nuisance than anything. Something on the order of $150 each. The lady MP that got them, waited till they were getting back to their car to leave, and told them she'd been watching them for over an hour. Which sort of irked my friends because .... in their mind ..... it was like ....... "if this is so friggin' wrong, why didn't you come over here and talk to us, or give us the ticket, an hour ago??". I suppose they too could have fought it, as there was no posting to the effect, and I suppose they might have argued they didn't have anything over 50 yr. old, or were looking for their boyscout ring, etc.. But when they saw the amounts of the ticket, it was just easier to pay and not fool with it.
But barring that 2nd incident (which admittedly they could have "figured"), and not counting the first incident (which my friend was not even detecting, and could have successfully appealed), I have not personally heard of anything.
I'm sure there *might* be a rogue incident someone could find, of someone getting jailed for hunting city sand-box somewhere. I mean, so too can someone probably come up with an incident of an over-zealous cop roughing up a motorist for nothing but a tail-light out.