I spoke to the city building department. The bad news is that they will not allow a gas pipe under the patio slab unless it is done per code which would require a vented sleeve for the entire length. That was the rule for many years at the county but then it was changed to allow it under open patio covers. LNG is heavier than air so it's not possible to reach a fuel/air ratio that will ignite.
I once was called to a fire that involved a commercial tortilla kitchen in a back yard. The stove was run on propane and propane is also heavier than air. A 100 gallon tank leaked and the propane was trapped by a house, garage and fence. The explosion busted out the house windows, knocked the garage of the foundation, broke the fence and killed the cook.