The Atrium note is an artifact in the Left Behind DLC chapter So Close, set in the present-day Colorado mall. It is in the atrium, to the left of where Joel and Callus are hidden, near the remains of a field surgery.
Regan and Private Ellis go into the loading dock for food and are attacked by a group of stalkers. Ellis is bitten.
After Ellis tries to end his own life, Regan knocks him out and breaks protocol: she ties off the arm, sterilizes a saw from the abandoned triage, gives him morphine and amputates above the bite, convinced it will save him.
Atrium recorder
The Atrium recorder is an audio artifact in the Left Behind DLC chapter So Close. It is in the tent near the crashed helicopter on the mall's upper level, the spot Ellie has been trying to reach all chapter.
Generator note
The Generator note is an artifact in the Left Behind DLC chapter So Close. It sits next to the generator at the refugee camp inside the Colorado mall, where Ellie has to restore power to reach the upper level.
Combination note
The Combination note is a required artifact in the Left Behind DLC, found behind the counter of Weston's Pharmacy in the Colorado mall during Back in a Flash. It gives the code for the American Princess store next door, where the key Ellie needs is waiting.
Crew photo
The Crew photo is an artifact in the Left Behind DLC chapter The Enemy of My Enemy. After Ellie leaves the helicopter, look in the corner of a dark room shortly before the hostile survivors show up; it is on a decaying body.