I found one last year. They're used for a magic trick where you change one coin to another. The one I found was made from a 1977 JFK Half and looks the top one in your picture.
I remember seeing halves at the Air Force Museum in Dayton with secret compartments. They were part of the Air Force OSI exhibit. If I remember correctly, I believe the fakes halves on display were once used by a spy ring to smuggle film.
I think I have found one, how the heck do you get it open without cutting it in half?
I got mine open by holding the edge of it in one hand and flikking the bottom of the rim with the thumb nail of my other hand. Turn it a bit once it starts to seperate and pretty soon it's 1/2 way open and you can grab with your fingernails and open it. At least that's how I got mine open. I figured something was up with the coin when I flipped it and it sounded funny. I saw the machined cut line with a loupe when I looked.