It sounds like they had a leak in the gas pipe and that the original line ran through the slab.
In a mansion house that I renovated from 1999-2001, I had a gas meter at the top of the driveway with a 230' driveway down to the house. Both sides of the driveway were lined with 50-year-old pine trees. There was a fireplace in the family room, an outside gas grill on the second-story deck, and a 450,000 BTU pool/spa heater. One day I found those beautiful blue/green irridescent flies near the spa heater, so I knew there was a gas leak around there somewhere and, using soapy water, I eventually found it.
Because of the long run under pine trees and under the slab and in the walls, I got a cost of $27,000 to run a new gas line. Considering that I was renovating for sale, that was not feasable and would have destroyed the symmetry of the landscaping on one side of the driveway because the pine trees would have to be chopped down. Options in my unincorporated area of the county provided for laying the gas pipe on the ground behind the pine trees and covering with 18" of soil, landscape bark, gravel, etc., so that the pipe was the required 18" underground. I installed semi-large boulders and built a mound of soil on them and installed shallw-root plants. When we got down to the house we attached the pipe to the bottom of the siding and ran it over to the pool/spa heater and up to the exterior grill and inside fireplace.
It originally was pretty ugly down at the house, but some good painting and landscaping covered up the ugliness.