Your meter feeds the whole house. There will be a shut-off in the house that shuts everything off, or outside the house that will feed water to a vacuum breaker that will be located about 3 feet above the highest head on the property. This back-flow preventer is in place to keep water that's in your sprinkler pipes from being drawn back into the domestic water system, spoiling it for everyone nearby. It has its own pair of shut-offs. Close one valve and it will isolate your sprinkler system.
Do you have a pool or a water softener? These often have chemical injection systems that use water pressure to work. A stuck valve or solenoid can make the systems flow full time.
There are many potential sources for leaks. The biggest are the toilets in the house. A bad flapper seal can blow through 20,000 gallons a year. That's just a simple dye test. Put some dark blue toilet bowl cleaner in the reserve tank and leave it alone for two hours. If the water in the bowl turns blue the seal is leaking.
Are you on a slab? Copper pipes embedded in concrete can crack at expansion joints or can be eaten away by chemicals from the inside. The lye in cement can eat copper from the outside.
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