Sewer gas odors frequently fluctuate and come and go due to barometric pressure changes, fluctuating wind speeds, and the varying evaporation rates of P-trap water seals. When low atmospheric pressure occurs outside, it can pull sewer gases up through household drains. Additionally, if a rarely used guest bathroom shower P-trap slowly evaporates over several weeks, it will allow sewer gas to escape intermittently until someone runs the water again. Similarly, strong wind gusts across an obstructed roof vent stack can force sewer gases back down through household drain traps. Verification: Pour water down unused floor drains and P-traps monthly to keep vapor barriers fully charged.
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