Sometimes if you need to find a leak you can hire someone with an infra red camera. It senses the temperature difference in the area where the leak is.
I just did one last week with our FLIR camera and found the leak in the main line in the yard. The line was 300 feet long. The temp outside was 35 degrees , where the leak was the ground was 54 degrees. Dug down and hit the muck in no time.
Works when the temps are 10-15 degrees or more apart.
Ground water is around 55 deg.
Good luck and follow Glenns advice.