I don't know why that ...... whenever this subject of federal land (of various types) comes up, that someone is sure to come on and say "you can't detect federal land". As if that is some all-encompassing rule or law.
It is simply not true that you can "never detect federal land". Perhaps there's certain TYPES of federal land, but you have to remember that not all federal land is the same TYPE. Not all federal (or state) land is federal (or state) "park" land. There are other levels of land that is federally owned. Road right of way is an example, etc.... Federal land can have many many variations of use. Military bases, BLM, eminent domain in-lieu of some form of future construction, libaries, post offices, etc... that are all not "parks", for which you may have found some sort of dire wording.
Here's a link to national parks with an express ALLOWANCE for md'ing. Beats the heck out of simple silence on the subject, or forever wondering if you're running afoul of some ancillary verbage, eh? I mean, you can't argue with an express
allowance, can you?

Yet lo & behold, it will continue to float on any such thread: "all federal land is off-limits". Why? I don't know.
National Forest Camping Rules
As far as the suffix to that allowance, that you can do it insofar as you collect "nothing of historic value"...... well ... gee, I don't particularly think that seated dimes and quarters are "historic". Do you? I mean, c'mon, seriously folks, do you really think someone is following you around with a calculator doing the math on the age of items you find the second you pull it out of the hole? For pete's sake put "those" items in your "other" pocket.
I mean, sure: Use common sense and don't tromp on sacred historic monuments, or go during an archie convention. But for run-of-the-mill middle-of-nowhere forests? Let's get realistic here.