Read a report and this report also may contain there were no reports of “rotten egg smell” which would indicate natural gas. Imagine there is checking to see if possibly chemical not added for smelling of leak, if it was a leak.
Hydrogen Sulfide (H2S) is nasty stuff with a very wide explosive range (4.3 to 46% in air), can be smelled at less than one part per million, and is dangerous for prolonged exposure above 100 PPM, and paralyze the respiratory system in short order above 500 PPM.
Low concentrations, had this been smelled, might have been displaced from the sewer system by a gas leak into the sewer lines, but, as you said, no one smelled 'sewer gas' (H2S) either.
For me, that would likely indicate natural gas accumulating in the building as the culprit.