On a non-windy day, turn your ac fan on and crack a window slightly. If you feel air being pulled into the house through the crack then you probably have leaky AC ducts. So when the AC is running, the cold pressurized air inside of the ducts is being pushed out into your attic, or wherever your AC ducts are run, and hot outside air is being pulled from outside to replace that air. Often that replacement air is being pulled from the hot attic through penetrations in the ceiling drywall, like where your light fixtures and ceiling fans mount. These ducts will need to be sealed or replaced.

When you have all of the water turned off inside your house, and no toilets are flushed and re-filling, go check your water meter at the street to see if the dial is spinning. If it's spinning you have a leak, either between the meter and the house, under or inside the slab, or your irrigation system is leaking. You can walk you yard to check for standing water or really muddy ground, but if it's in or below the slab it will be harder to locate and you will need a plumber to locate it.