To get there you need to be in the first part of the mansion level, then find this corner with 2 bookshelves, the one underneath the painting has the switch to open the secret passage, watch out for enemies, go down the latter that is in the back (not very far), there is 2 spider drone guys, juz kill em then use /noclip and walk through the door, and look into the security camera. :)

Near the end of the level, in the room with the mooving sculpture thing, there are some big pannels on the wall, with repeated pictures on them. in the bottom left, of the left pannel the picture is backwards, 'use' this picture, to open a secret door to the back of Elexis Mansion. Out here you can find the Bio Shield thing (Secondary Objective) Being guarded by a big mutant thingy. Also, you can find the key to the Janitors (Level Secret) room in a dark corner near the mutant.
John Garrod