Pass Card

In Fallout 3, s are items that are randomly encountered.

Locations:
There is no place where a pass card is guaranteed to be found. They are dropped by ghouls or centaurs, and may also be found on trogs if The Pitt is installed.

A ghoul has a 1 in 80 percent chance of dropping a pass card of either color, A centaur has a chance of about 1 in 170, & trogs have an even smaller chance of about 1 in 240.

Notes:
The cards seem to have no purpose. (they are not accepted by Protectrons in lieu of metro tickets!)

They could be a reference to the area from Fallout 1 called The Glow. You needed red, blue, and yellow pass cards to move to the lower levels, or its an homage to fallout 2 where you needed red and blue pass cards to move around a ghoul run nuke plant