you have to get in with no clip, which i think is the first and only secret in QL official maps (made by id) which involves no clip to enter.
there are various secrets in the training area's.
hell there's even a bunch of them on the crash course.
but this secret is my favorite. becuase it's an entirely new secret in a new map.
and i found it with luck ;-) the map seemed so beautiful with the outside thingies that i noclipped it. and after accidantelly falling trough the floor, i found that room.