Water heater on pilot
Many gas appliances have a "Pilot mode" where they will just burn a pilot light, and ignore the thermostat setting. Typically a 3-position control reading "Off-Pilot-On" and it sounds like yours is probably of that type and was in "pilot." 

Turning this into a hunt for a gas leak is just plain illogical. 

Almost certainly what the tech flipped from "Pilot" to "on" (and was internally laughing all the way to the bank about.)