I notice you say "probably", so .... let's clarify : It's an oft-repeated wive's tale that all state and federal land is off-limits. You might find some state's parks, that do indeed have a rule of "no md'ing". But there are other of the 50 states (like CA, for instance) that are silent on the subject. Hence, not prohibited. In that case, it might only fall under their cultural heritage boiler plate wording. Or "harvest and remove", or "alter & deface", blah blah.
So in CA, for instance, we hunt state parks and state beaches all the time, w/o issue. Assuming, of course, it's not an obvious historic sensitive monument. Or assuming, of course, there's not an archie convention going on.
As for federal: There's actually EXPRESS ALLOWANCES for md'ing in NFS and BLM (beats the heck out of silent on the subject, eh ?). But, sure.... cultural heritage ARPA comes into play. Fine then: Just don't find any coins older than 50 yrs. old (hows your math skills ?)
Do you realize that there are some highways and through-roads, that go right through federal parks. Like Hwy. 14 and Hwy. 191 that go through Yellowstone National park. How then, is someone who is traveling east-to-west, supposed to cross the state , if they (gasp) have a detector in the trunk ?
I have often heard this claim of "busted for merely having a detector in the trunk" type assertions. But when you ask someone for an example, none are ever forth-coming. If you say you know of a true example, I would love to know more details. Me thinks something else had to have been in play. But if I'm wrong, and someone was "roughed up" for merely having a detector in the trunk on a through-fair drive, then here's my thoughts on that :
Fluke stories will ALWAYS exist in life. Ie.: so too..... if I searched the newspaper headlines long enough and hard enough, can probably find an example of someone who got pulled over, roughed up, car confiscated, jailed, fined, etc... For nothing but a tail-light out, from an over-zealous cop.
Sure, flukes happen. But we don't let those "flukes" rule our lives (or "stop us from driving", etc...). I even have a newspaper clipping of a guy who got a ticket for eating a hamburger while driving . The cop called it "distracted driving". But .... I'll bet that clipping or fluke event doesn't stop you from eating a burger while driving. You attribute it to exactly what it is: A fluke. Not saying to "throw caution to the wind", but .... I just have a feeling of this "felony for merely having a detector in the trunk" is going to be in the fluke category.