If you are a treasure collector then you would be looking for the first treasure under a tarp down the stairs when you first gain control of your character, and to your surprise a guide was wrong for the first time ever. (Audience trembles) Explanation. The first treasure on XBOX 360 was under the tarp down the stairs but on PC has since been moved. After travelling up the staircase and dispatching the Arabs you will proceed up the stairs into the building where you will receive a popup talking about hidden treasures. (How ironic) You then turn immediately to your right and move to the far corner on the left. There is a circular basket that enquires you open it. Inside is our lost treasure. No idea why the devs made this move but they did.
P.S. When you start and head all the way down both stairs to the left there is a guy resting on a tarp with a pickaxe. Once you receive your guns if you head back he will no longer be there anymore.

TL;DR - First treasure is when you enter the building after killing the Arabs. Just head right and look for the circular basket.