Spec Ops: The Line

At the start of the game make your way through the abandoned cars. You will shortly afterwards stumble into an ambush by the "insurgents".
Fight your way through with your squad. During the fight you will be given a brief introduction into issuing orders to your men.
Once the fight is over you'll hear someone from the 33rd Division call on the radio for the first time. That's when you have found what's been lost and gained yourself a trophy for doing so.

Unmissable storyline achievement. After you complete the prologue (with the helecopter), you will be walking along a deserted highway. Halfway through this first chapter, you will find spot a person. The trophy will unlock shortly after your first fight.