OK - here goes...
Looks like the map author either intended this to be a cooperative effort, or just difficult to get it open, or both.
Here is the trigger's location as opposed to the door...When you are facing this "secret" door/passageway on the 2nd floor, (below the 5 pics of Angel, Bragtnik, ButtKicker, Reaper and Yugi), turn to your Right (90 deg.) looking towards the corner and the book shelf.
Go straight to that book shelf wall in the corner, then turn right (90 deg.) again and go along that wall towards the bookshelf.
When your are just getting near to the back of the bookshelf and pressing to and along that wall behind it, you will see a tiny little cube shaped button, (about neck high?), that has the same texture as the wall does so it is hard to see straight on to it.
When you are near to or bump? that button, the words "Door Open" will appear in your message window in the upper left on your screen.
The problem is, you will not have enough time to run over to that open door before it closes again.
What I did was drop a trans just outside that door in the little hallway/tunnel - run over to the trigger button by the bookcase - when it opened I tele-ported back to the door and then went in to the room behind.
This takes some time though and if in a real game it could be pretty dicey.
There are more things I want to look at in this map when I get more time.
It appears that the secret door you are asking about here is the only one triggered as it is though, but not sure.
Enjoy!
