On my recent section hike through Shenandoah I encountered several PATC cabins. I believe there are seven in Shenandoah and it looks like there are more on other parts of the trail maintained by the PATC. I didn't stop at all of them, but the ones where I did stop it was pretty much the same: nice cabin about the same size as a hiker hut, sleeps 6-8, near a spring, with privy and picnic tables nearby, and open space that would be good for tenting.
They're definitely not a secret - they're labeled on the map, and they were all on clearly marked side trails within a few tenths of a mile from the AT. In fact, at least two were visible from the AT. The ones I stopped at were all locked, which wasn't a big deal, but the big deal was that they all had "NO CAMPING" signs. This really irritated me - enough to warrant starting a thread.
I understand that the PATC maintains the trail, they own the cabins, and they have the right to keep hikers out. But to assert that they have the right to keep hikers from camping nearby - in what would otherwise be a perfectly legal (and ideal since they're near water sources) tenting sites is ludicrous. In fact, IMO it's really against the spirit of LNT since it encourages hikers to clear new tent spaces elsewhere rather than using areas that have already been cleared.
The other thing I noticed at the cabins where I stopped is that they were all unoccupied, while the hiker huts were near or at capacity (including tent sites) pretty much every night. The huts aren't always ideally spaced, and while stealth camping is legal in Shenandoah, there aren't a whole lot of established tent sites near the trail. This often leaves hikers with the choice of a 13 mile day vs a 26 mile day.
Perhaps the intent is to get more hikers to pay to use the park camping facilities, but that clearly wasn't happening. Most of the hikers I saw laughed at the idea of paying even $20 to get a tent site at one of the campgrounds. So bottom line is that if they just allowed camping near the PATC cabins, it would reduce the impact on the trail and especially at the huts, and allow hikers to have better options in terms of planning daily mileage.