Hitman: Absolution - Birdie's Gift

You will find it by using our walkthrough for this mission. The evidence is found in the bunker behind the shooting range. You need to go behind the front desk and grab the key card while the shop keeper is distracted. Go through the back room, walk through the trenches and then watch for the door on the right. Open the door and inside the bunker you'll find your evidence on a shelf.