Pass Card

Yet unknown items who are dropped by ghouls or centaurs. There are currently two 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.

(Edit: Actually, they are more likely pass cards for the Blue and Red subway lines that you see on subway maps and when you are actually in the subway tunnels.)

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