Today I went to my backyard through my basement door doing some grill and saw fresh water coming out of a drain pipe that's mostly buried underground. It's sunny and hot outside. Though very odd, I sensed a pipe leak somewhere that happens to be close to the drain pipe, which is likely broken too. The street main is on the front of my house and the service line comes straight into the house inside the basement. After I turned off the main valve and cut off the water supply to the entire house, I saw the water meter was still running fast. But I could not figure out how the fresh water could leak from the backyard. Is it possible that a second water line comes off the service line between the water meter and the main valve inside the house, and then goes around the house from outside to the backyard underground? I am not the 1st owner of this house and cannot imagine someone would come up with this kind of design. And I cannot locate either another control box or a valve around the house. I have since turned off the water inside the house and the whole family will suffer tonight and tomorrow at least. I wonder, should I dig along the drain pipe that's deeply underground until I locate the leak? It seems to be a huge undertaking. Or should I try to locate a control box or valve that's not controlled by the main valve, even though I do not know if there is one?
I knew there was an irrigation/sprinkler system in the backyard done by the previous owner, but I never used it. To my knowledge it seems to be above ground, using the water hoses. Plus there is no sign of leak in the area where the irrigation hoses are laid.
Can someone provide tips/ways to find the leak?
Thanks very much in advance.