Pass Card

In Fallout 3, pass cards are items that are randomly encountered. The cards serve no other purpose than causing the protectrons to help you fight nearby enemies when in the associated subway lines. (Red, Blue) When you obtain one, a room nearby most likely harbors the Protectron and its encasement. When activated, the Protectron asks you for a "Pass, please", and if you have one in your inventory, he follows you and protects you from enemies.

Locations
Pass cards are located in random places throughout the Capital Wasteland. They are dropped by ghouls, centaurs, and may also be found on trogs if The Pitt is installed, since Trogs were once civilians, they have the chance of carrying the pass card.

A ghoul has 1.5% 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%. Ghouls are known to live in any Metro Station, located in Washington D.C., so ghouls have the chance to carry the card.