Successfully complete one of the following tasks to unlock the corresponding item inside a shopping bag behind the air duct in the Security Room:

Ammo Belt: Do not miss with a machine gun.
Arthur's Boxers: Survive for at least 7 days.
Hockey Mask: Photograph all PP Stickers.
Laser Sword: Survive for at least 5 days.
Mall Employee Uniform: Answer all calls from Otis.
Mega Man Boots: Get the true ending without being knocked out.
Mega Man Tights: Defeat at least 10 psychopaths.
Prisoner Garb: Steal the convicts' vehicle.
Pro Wrestling Boots: Break at least 100 items.
Pro Wrestling Briefs: Defeat at least 1,000 zombies barehanded.
Real Mega Buster: Defeat at least 53,594 zombies.
Special Forces Boots: Defeat a least 15 psychopaths.
Special Forces Uniform: Repel a helicopter.
White Hat: Photograph at least 50 survivors.
Unknown: Get at least 50 survivors out of the mall.