I have an anti siphon, frost proof faucet that leaks through a hole (vent?)in the stem when the water is turned on. Can this be repaired or must the whole unit be replaced?
Yes, the anti-sipon valve can be repaired/replaced. Rubber washer inside. Find out make/model and find replacement washer.
Joe is correct and its a common thing found on home inspections.
The only time I am able to find it is if there is pressure on the sillcock…like a water pressure gauge for example.