Near the switch raising the staircase at the west of the map, there is a high alcove containing a computer map and a soul sphere, with a lighter metallic floor in front of it. Look toward the west window to find a switch on the wall outside. Shoot at the switch while standing on the lighter floor to be carried up to the alcove. (sector 199) Once you enter it and grab the items, press on the metal wall when it finishes rising to lower it again. Note that you can only trigger the platform-rising switch once; if you shoot it while not on the platform and don't get on it in time, you will be unable to access this secret.
Once you get the yellow keycard, return to where the switch lowering the pillar it sat on was to find that it has lowered, allowing you access to combat armor. (sector 203)
The "yellow" key-locked barrier (linedefs 1772-1775) is erroneously flagged as requiring a red key (linedef type 33 instead of 34,) rendering the yellow keycard useless.