Well, it's Melina's office; she must know. So call Melina and ask for it --
-- but not right away. For some reason, you have to go talk to Niobe first.
Niobe asks you get her access to the coin display. Call Melina about that, and now she'll tell you where the filing cabinet key is.
Melina says she keeps it in the office, in the puzzle box with the animal figures in the open bookcase next to the filing cabinet. Of course. Who'd leave a key where you could actually, you know, use it?