How do you tap the bodies for items when you tranq them?

When you want to loot a body, stand by the head. Make sure you are actually standing, crouching doesn't work. A prompt to press up on the D-pad should appear, do so (although if you're standing in the right place, the prompt doesn't matter). Snake picks the body up, then press up again to put the body back on the ground. An item will fly out and land beside the body. I've found that if a guard is unconscious/sleeping, he gives you two items, one for each time you pick up and put him down. Dead enemies give you one, usually ammo.