A full outer join is used when you need to return all rows from both the left and right tables. Where there is no row match in the other table then a row of nulls is returned.
SELECT E.LastName, L.LocationID FROM Employees E Full OUTER JOIN Location L ON E.LocationID = L.LocationID
This should include, in a single list, employees that have not been allocated a location and those locations that do not have any employees. The list can be filtered futher by checking for NULLs on either table.