Hitman: Absolution - Countdown

Blackwater Roof

This evidence is in a side room locked with a keycard. When you get to the room that requires you to break a chain with a sledgehammer, search the room for a keycard. When you have it, head back out onto the previous section of the roof and look to the right. On that wall should be the door you are looking for. The evidence is in the second room on a box.