Pass Card

In Fallout 3, Pass Cards are items that are randomly encountered. The Cards serve no purpose, save for being a light source of caps.

Locations
Pass cards are located in random places throughout the Capital Wasteland. They are dropped by ghouls or centaurs, and may also be found on trogs if The Pitt is installed.

A ghoul has 1.25% chance of dropping a pass card of either color. A centaur has a chance of 0.6% and trogs have an even smaller chance of about 0.4%.

Behind the scenes
They are most likely a tip of the hat to the 1993 first-person shooter video game by id Software called Doom in which the player had to collect Red, Blue, and Yellow pass keys to advance. The keys were always held by nondescript zombies.