Inside the Military Prison (either at airfields or other sites with them), as soon as you enter the main double doors that swing outward, on the first floor, run toward the back wall ahead of you (passing by the prison cell), and as you get close to the wall do a jump. As you jump into the wall, you will enter to the other side of it. Inside from here, you can see through a number of walls, but you will be invisible to other people either on the 1st or 2nd floors. The only way to exit though is to run toward the room with the two desks, and as you get close to the wall then do a jump.

You can also repeat this with the 2nd floor. It will the same room up above the first area. To get into the area on the 2nd floor above the 1st floor area, you'll have to do a running jump as well. The only way to exit is to run & jump toward the room with the bunk beds.

If you're either in the 1st or 2nd floor areas, you can see through walls, and no one can see you. You can't shoot through the walls, but you can be shot at by other players if they are in the room with you, or above you, or in an area opposite the main doors that has a door to it but you can't enter.

So if you were to face the main two doors upon entering, look to your left and there is a door there. I don't know how yet, but I saw players enter through that door or wall. No one can normally enter that area, as it is a hollow space with nothing inside, however I saw people from my vantage point (I was on the 2nd floor glitch space), and saw heavily armed players teleport there.

I wasn't able to shoot them, but one guy with a mosin shot at me, and I died.