Three day investigation on this including on-site inspection including county water department (in my presence) plus discussion with plumber leads me to believe
something I've not heard of...water theft. Either someone with a 20K gal tank drove up while no-one was home and hooked up to exterior outlet, filled up and drove away or also under cover of dark when when no-one there ran garden hose several hundred feet and filled their swimming pool. I want to ask my insurer if my policy (which includes theft) covers this type of theft. My problem is that a few years back it was documented that any enquiry about one's policy, in many cases, led to increases in premiums some time later. Rather than for that to happen I would rather bite the bullet in this incident and pay than to risk increase in premium... I would but a lock on exterior outlets.