Pass Card

Pass cards are yet unknown items that are randomly dropped by ghouls or centaurs. There are currently two different passes discovered. It may be an easter egg referring to Doom, where keys were represented by blue, red and yellow cards to open corresponding doors. More likely, these colors correspond to the Red and Blue subway lines beneath the city.

It is also possible that they are like the metro tickets the player can pick up, and can be used when activating security protectrons in metro stations. The protectrons check for tickets or passes, having one means they ignore the player and shoot those who don't (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 Cemetary

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

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