I live pretty close to Jefferson National Forrest, it's not a National Park, it's forrest. I've hunted for deer and smaller game in Jefferson Forrest, never had a problem.
Papa, if it were ME, I would go detecting...like you mentioned, all the ranger stations are closed. If by some slim chance you're stopped (which I don't think you would), I would explain that it's a national forrest, not a national park, you didn't know, and you'll be happy to leave.
I can't see a problem. I hunted alongside US 23 leaving Big Stone, along the four lane going into Tenn, all that area is Jefferson National Forrest. Numerous Caves can be seen from your car while driving down the road (US 23) from Big Stone Gap VA, to Kingsport, TN. My relatives have been inside nearly every one of those caves along 23, I think the one that looks like a small slit in the rocks opens up like an airplane hangar once you squeeze into it about 30 feet (according to my cousin).
Anyways Papa, open up this link and scroll down to Metal Detecting in the National Forest, it states very clearly that it is ok.
Hope this link helps:
http://www.fs.fed.us/r8/gwj/general_info/index.shtml
Well, that's my .02
