AFAIK the only storage requirement has to do with the location
of the firearms, not how they're physically secured. To quote 27 CFR Section 478.50:
Sec. 478.50 Locations covered by license.
The license covers the class of business or the activity specified in the license at the address specified therein. A separate license must be obtained for each location at which a firearms or ammunition business or activity requiring a license under this part is conducted except:
(a) No license is required to cover a separate warehouse used by the licensee solely for storage of firearms or ammunition if the records required by this part are maintained at the licensed premises served by such warehouse;
subsections (b) thru (d) omitted; they relate to where firearms may be sold, not stored
In other words, the firearms are supposed to be located at the address listed on the license
unless they're elsewhere for a legally permitted purpose, such as personal use, legal loan to a friend, being transported to a gun show, or whatever.
My understanding is that the term "warehouse" is not clearly defined and is somewhat subject to interpretation. However, since the law does
say that the building should be "...used by the licensee solely for storage of firearms or ammunition..."
, you probably shouldn't store the firearms at, say, your office or your aunt Mabel's house, since it could be argued that you visit those places for reasons besides firearms and ammo storage.