I've detected on B.L.M. lands here in CA, and no one ever said anything to me

But then again, maybe I didn't ask enough questions, far enough up the chain, of enough bored bureaucrats.
Probably the technical answer (which would be true of perhaps any public place you might "ask" questions, phrased in *just* the right way to get things morphed to apply to you), is "no". I mean, if you ask long enough and hard enough, with the right mental images portrayed, you can probably get a "no" for the most innocuous sandbox you come to.
Since BLM is federal, I'm sure ARPA would apply. But then again, is anyone really standing over your shoulder, with a calculator, determing if your coins are over 50 or 100 yrs. old or not?

And in any case, just tell them you're looking for meteorites, which are not man-made (they're from outer space). Thus it won't fall under any cultural heritage wording. Or ...... of course ...... you're looking for that boyscout ring you lost there when you were a kid, right?
But seriously now, if the BLM land you have in mind, has some sort of obvious historic monument right there, and you plan on going right in the middle of an archaeologist convention on the site, then yeah, you'll probably get bothered. But if you're going to be out in the middle of
nowhere .........