Frost-free silcocks have the control way inside, so that the part that goes through the wall can drain. Most versions won't do that unless you remove the hose. So, the only solution is to replace the whole thing, as it is likely split. If it leaked all the time, it might be rebuilt, as it would be the valve, but only leaking when on is a big flag that it is split. An alternative replacement is maybe a Woodford Model 30, which will drain, even if a hose is inadvertently left on it.