Top Secret Communications Center
When my son was in the Air Force, my wife and I visited quite often. On our first visit, we were allowed inside this top secret Communications Center, but everything in sight was covered up so we could look around everywhere -- Heck, even the toilet paper in the Men's room was disguised.
Anyway, at the exit, there's a sign above the door, which reads: "You have been exposed to Top Secret Material. Please destroy yourself before leaving the building."