We miss you!  It is probably a leak in the pipes leading to sprinklers. It’s very easy to test for it and not too expensive. Then they can bypass the leak and change out the pipes.  Even worse, find out where it happened because a water leak can damage your foundation if it’s under the house.

Yes it could be expensive. Let me know what you find out.

Good luck, hope its a cheap quick fix.  

Gotta be a leak.  Hope they. check the main water line that serves your entire neighborhood.  We had a main pipe in Scottsdale get a leak and they had to tear up the street!  If you only use under $50 a month it's unlikely one neighbor could steal $200 in one month as an example.

My DH can tell when our outside water is running.  Ask plumber about a lock for your outdoor faucet.  We had one on ours during one bad neighbor time who used our water to water his horses.