Explain the hiding spot of the Franks in a short way.

From the text:

Here's a description of the building. The large warehouse on the ground floor is used as a workroom and storeroom and is divided into several different sections, such as the stockroom and the milling room, where cinnamon, cloves and a pepper substitute are ground. Next to the warehouse doors is another outside' door, a separate entrance to the office. Just inside the office door is a second door, and beyond that a stairway. At the top of the stairs is another door, with a frosted window on which the word "Office" is written in black letters.