When double carrying two weapons and jumping over a fence with a gun in your hands causes the player to do the "put weapon on your back"-animation.
Your primary equiped weapon then will disappear. You are not able to put anything in your hands anymore (like a small hatchet for instance). When you do some other action like "bandaging" or "drink soda", the lost weapon will then lay on the ground in front of you as you finish the before mentioned action.