The Museum Storeroom can be accessed through the door at the staircase in the Staff Barracks. Note that it can't be reached via the Curator's Guide.
To the left of the stairs the Distilling Station is located. It can be used to brew drinks for the Reserve Vintages display, which is located in the two cabinets next to the station.
To the right there is a large Wine Rack, which is connected to your supplies via the Craftloot system. Behind the rack, there are random boxes, barrels, and other unused items. This is the location of the Macnarian Portrait during the quest Night at the Museum.
Straight ahead from the entrance, there is a table with a Strange Button and a model of the Classic Museum. On this table the "Mus-Tard-Jar" from BadGremlins Collection can be placed, although it does not count as a museum display.
Left of the table there is a Debug Portal that gives access to the Debug Room. The key to the portal can be found on the Key Rack at the wall above the table.