In Uncharted 3: Drake's Deception, I jumped onto an enemy from a ledge and killed him quietly. After doing so, i picked up a grenade and a little blue ninja-looking icon popped up along side it. What is the meaning of the icon? Does it only indicate that I made a stealh kill, or does it mean the item I picked up is somehow different than another item?
You get a slight bonus in ammunition gained when you pick up an item with the mask because you've killed someone with a stealth kill.
In Uncharted 2, I believe it meant extra ammo because of a stealth kill.
In Uncharted 3, it means that you get a better than average weapon for a stealth kill. If you look closely, when the enemy is alive he may have a Arm micro or Ak-47, but when you ninja him (stealth kill) he may drop a tau, T-bolt, or an explosive of some sort. In some cases it's just a grenade so you can't really tell the difference.