The gas company's test probably will detect high levels of methane, as well as their gases. Unfortunately, one of the odors from sewer gas is often a sulphur compound that can be detected by the human nose at levels that are hard to detect with equipment. The best approach if you suspect sewer gas is to check for possible sources. Dry traps in floor drains or a blocked vent stack are likely sources. Or, ask a plumber to check for leaks.
Plumbers often have gas detectors that can help locate gas sources.
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