Is your water heater down in the basement and if so does it have its own dedicated drain for the expansion relief?
My early 2000's condo has a dedicated 1.5" drain line for the water heater blowoff and a/c condensate line, took me a year to figure out why I kept getting sewer odors towards the end of winter. No a/c condensate and water heater has expansion tank so the trap was just drying out all winter long. I now know to go open the relief valve for a few seconds a few times during the winter. It did not help that the drain is tucked way in the back behind the water heater where the base of the heater is mounted at my eye level (above a dryer), would never know there was a drain back there unless you hopped up on a ladder and looked around.