I have a three zone system and there is a leak somewhere. Here is what I have done.

1.) From the street shutoff to the backflow valve it is not leaking because I can turn off the flow at the backflow valve and the meter shows no usage.
2.) I dug up the two zone valves that are side by side 30 feet up a hill from the backflow valve. No leaks in this area.
3.) I looked at all the heads from the three zones to see if a zone valve might be leaking but see no water draining out of any head. Or any water in the head.
4.) The third zone is 100 feet from the two zone valves that I dug up. This line runs under the drive way. I suspect that the leak could be in this line. Or it could be from the backflow valve to the two zone valves, around 30 feet of pipe.

I have no water on the surface along any of the piping. I used a sharp rod and moved along the lines where I think they are based on a sketch I did when the system was installed. Not the best sketch but better than nothing.

Does anyone have any ideas on how to locate this other than to start digging up the supply lines to the valves. I guess start with the 30 foot section to the two zone valves and then tackle the 100 foot line that goes under the driveway.

Or I cut the pipe at the two zone valves and put in a shutoff to isolate the 30 foot line from the backvalve and if that show good then move that to the 100 foot line. Then I start doing digging in the middle of the 100 foot line and adding a shutoff and keep moving based on what I find.

Or I thought of some kind of listening device that might be able to hear water movement in the pipes. The meter is showing the little dial moving in a circle about once every 2 seconds.

Thanks, Tom