In Fallout 3, Pass Cards are items that are randomly d ropped by ghouls or centaurs.
There is no place where a pass card is guaranteed to be found. A ghoul has a 1 in 80 chance of dropping a pass of either color; for a centaur the chance is about 1 in 170. The cards have no purpose: they are not accepted by Protectrons in lieu of metro tickets.
Its likely a nod towards games like DOOM that used similar cards to open doors to the next area