Accessed in Appearance → Customize → General settings → Hidden sidebar.
It is always available sidebar(It is only active if it has some active widgets in it). User can view it by clicking on icon in header tool icons.
Good for some special menus or other tips, information for user.
You can set here few things about Hidden sidebar appearance.
How to remove it?
If you don’t need this sidebar and icon in the header, please remove all widgets from the “Hidden sidebar” in the widgets settings.