Security Pass is one of the key items that is required to progress through a level. Two Security Passes can be found in Quake 2.
In the Security Complex, after opening the pyramid, the door to the Grid Control requires a Security Pass. It is found in the Torture Chambers, where a captured prisoner will drop it when killed.
Another Security Pass can be found in the Lower Palace. While it is not required to complete the mission, it grants access to the Yellow Force Field Control Switch, which, in turn, deactivates the Yellow Force Fields. Not only does this eliminate the need for backtracking through the Upper Palace and Lower Palace, but it grants access to a secret area.