Shiv doors are locked rooms full of supplies that open only when you spend a full shiv on them. Opening every one in The Last of Us Part I earns the Master of Unlocking trophy.
The Outskirts: a white door after the first clicker in Downtown, and one in a cafe in the Museum. Bill's Town: two in The Woods, one before boosting Ellie over the fence and one round the corner from the alley where Bill's trap blows up a clicker. Pittsburgh: one beside Rivers Cafe on the bookstore's second floor, one marked with a red X near the hunters on the river container, one in the back room of the HS&L Credit Union, and one across from the stairs after meeting Henry and Sam. Tommy's Dam: beside the stairs on the way up to the dam. The University: at the end of the science building hallway. Lakeside Resort: the wooden gate behind the motel. Bus Depot: down the right-hand tunnel past the workbench. The Firefly Lab: next to the information room on the hospital's sixth floor.
Always keep a spare shiv in your inventory.