Go on a jeep and drive it across the river and park it in a slanted position in the north west corner of the map.
There will be space in that corner.
While you're still in the drivers seat, get out and you will be able to go out of the maps border.
As you look south there will be a rock, to go in the rock you need to run and jump then you wiil be able to get in it.
As you are in the rock you will be able to kill people without them seeing you, but you can only kill people in that area.
Be careful and do not walk into the big space ahead of you because you have will to respawn. ENJOY.