public ActionResult SignOut(int?id)
        {
            if (id.HasValue)
            {
                var employee = EmployeeServices.GetUser(id.Value);
                if (employee == null)
                {
                    throw new ArgumentException(string.Format("No employee found with an id = {0}", id.Value));
                }

                SecurityServices.SignOut(employee, base.LocationId);
            }
            else
            {
                var signedIn = InvoiceServices.GetSignedInEmployees(base.LocationId, forceRefresh: true);
                foreach (var e in signedIn)
                {
                    SecurityServices.SignOut(e, base.LocationId);
                }
            }
            var employees = InvoiceServices.GetSignedInEmployees(base.LocationId, forceRefresh: true);

            return(PartialView("_AvailableEmployees", employees));
        }