I am going to assume that this sprinkler is leaking when the zone is off? Is it a continual leak far after the zone is shut off, or right after the zone is shut off? Typically on systems that do not have drain valves on the line, the excess water in the pipes will drain out of the lowest sprinkler head, typically at the end, after the valve is closed. However, if the sprinkler is consistantly leaking far after the valve is closed, it is very possible that you do indeed have a valve issue. Without digging up the entire yard to try and locate the valve, you will probably either have to rent some equipment, or call in a professional. There are devices in which you can hook up to your controller to make the valve solenoid vibrate loud enough that you can hear it above ground and easily locate it. This is by far, if you can get ahold of the equipment, the easiest route. You can also try a metal detector, but you probably won't have much luck there.
Tony Posey
Ridge Run Landscapes