The Antique Colombian Table Knife is the 9th treasure in Uncharted 3: Drake's Deception.
The Antique Colombian Table Knife can be found in Chapter 2 - Greatness from Small Beginnings.
When Sully first meets with Marlowe, you will find yourself on a roof with the Hotel Arrecife sign in front of you. When you jump to the sign, climb up to the top first. The treasure is in the "O".