Where the gas piping comes up from the ground into my house, there is a smaller "slave" pipe with a rectangular valve that loops back into the ground. I have accounted for all indoor appliances (fireplace, stove/oven, water heater, dryer, furnace) via the piping running through the attic. I asked the guy from the gas company when he was there flagging the yard, and he suggested it might be for an outdoor grill hookup. Problem is, I don't see how a grill could be hooked up to it. The entrance looks like this (fixed width font required):
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