L.A. Noire Upon Reflection first mission walkthrough what clues to look for in the alleyway crime scene?

Answer 1

Proceed through the alley with your back to the car.  On the right about midway down is a newspaper, around the corner on the left if you look up you'll see a reflection of the pistol.  The hack threw it up onto the roof.  You'll have to look around for a drain pipe to climb up to retrieve the weapon.  Those're the only two things that you absolutely have to have to complete that first segment.