In Extra hard mode, after you protect the little boy that leads you to the warehouse, you will kill a guard in the final house the kid goes to. That guard drops a keycard. Pick up the keycard and it will do a cut-scene and put you outside the house.

You have to turn around and go through the door you just exited. You will see a lone item sitting under the staircase. Pick it up. It is called the "Toy Gun". Upgrade for 15,OOO gil to get the "Silver Toy Gun", and again for 35,OOO to get the "Golden Toy Gun". A final upgrade for 2OO,OOO gil will unlock the "Ultima Weapon". It basically kills anything in one hit.