when you hit the main url . Odoo call the function of
 
@http.route('/', type='http', auth="none")
defindex(self, s_action=None, db=None, **kw):
    return request.redirect_query('/web', query=request.params)

that is in the controllers/main.py file in web module that function redirect to you /web url after checking the user session if not user session it will redirect to web/login.
in web/login after users credentials checking the user redirects to home page or back to it if credentials fails.
In your case check the web/login may be inherit in some module.
or you can refer to this

this