Pass Card

Pass cards are yet unknown items that are randomly dropped by ghouls or centaurs. There are only those two cards existing in Fallout 3. It was rumored to be an easter egg referring to Doom, where keys were represented by blue, red, and yellow cards to open corresponding doors. In fact, after seeking in bsa datas there is only a blue and a red one in meshes. More likely, these colors correspond to the Red and Blue subway lines beneath the city.

The protectrons check for tickets or passes, having the red pass will make the protectrons help you whenever on one of the red line stations, if in possession of the blue pass the protectrons will help you out on the blue line stations by killing enemies found nearby (ie. feral ghouls and raiders).

Drop places:
Red pass card - On a Ghoul inside the Museum of History Lower Halls - On a Ghoul next to Jaime at the bottom of Dunwich Building (Virulent Chambers) - On a Centaur in the vicinity of the church at Hallowed Moors Cemetery - On a Feral Ghoul in Metro Central - On a Ghoul in Bethesda Ruins - On a Centaur in Chryslus Building - On a Ghoul in the Irradiated Metro - On a Feral Ghoul in the Collapsed Car Tunnel (Dupont Circle) - On a Feral Ghoul in Warrington Tunnels

Blue pass card - On a Ghoul in The Taft Tunnels - On a Ghoul in Metro Central (At the top level of the junction) - On a Ghoul in the Dunwich Building - On a Glowing One in the Red Rocket Tricycle factory - On a Glowing One in Warrington Trainyard - On a Centaur in Georgetown - On a Glowing One while escaping The Jefferson Monument in the main quest, near the end when you encounter the Brotherhood of Steel Paladin (character) fending off Feral Ghouls, follow where the Ghouls are coming from - On a Feral Ghoul in Warrington Tunnels

NOTE: The places are random, but verification from multiple persons is required.