Piping to a sprinkler system are almost never run under the slab of a house. Unless, there is an addition to a house and a very poor caliber contractor is pouring the concrete. Usually, the sprinkler is put in after the hard boundaries are in like fences, slabs, pool, patio. Often when an addition is made the the property the irrigation system is modified to fit the new footprint. If you checked the meter correctly and there is no leak present that means that the only time you could have a leak is when the sprinklers are in operation. How long did you look at the meter? You should watch the small triangle in the face of the meter for five minutes and see if it moves.
If you need a rainsensor installed, you can do that to save water and also check your schedule. You should be 1 or 2 times a week, and your minutes should be reduce to fit this time of year. If you need more help, check out my web site.
www.sprinklerfix.com