There's a drawer on Mulder's filing cabinet that has a separate lock from the other drawers. The key is on its own ring, locked inside a lock-box in a safe. The key for the lock-box is inside a different lock-box, one that is kept under Scully's name at the bank. The key for the that box is locked in a third box in Skinner's safe, and the key for the third box stays secure the Gunmen's lair, though he has never asked Byers where it's kept.
Inside the drawer, Mulder keeps his biggest secret, all filed far more neatly than Scully or Skinner could ever imagine he's capable of. The dark green folders are organized by month, clearly labeled on the top, and inside each green hanging folder rest several manila folders, labeled by day of the month.
Inside these manila folders, Mulder has pictures. Polaroids, regular film shots, and print-outs from a digital camera. Line drawings, sketches, and almost professional-grade portraits. Some of them feature a man, sometimes smiling at the camera, sometimes seemingly completely unaware of the capture. Others are of the same man, younger - as a child, as a teenager, as a younger man. Still others are of two men together.
Only one of these pictures lives outside the drawer, taped to the inside of Mulder's telephone. When the nights get long and the bed gets cold, he pulls out his screwdriver, takes the phone apart, and stares longingly at the shot of him and Krycek, arms wrapped around each other, caught up in a tender kiss.