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?

How do I open the box with the animals?

The left drawer contains the key for the coin display and a sticky note with the computer password. The right drawer contains the labels for the vase display and timesheets for the staff.

You don't need to memorize those timesheets, but you should flip through them once to trigger a question for Melina later.