Since we repaired the floor drain in our basement, the smell of sewer gas has thankfully gone away. But now we have a new, mysterious odor that pops up seemingly without any pattern: what we can only describe as "wet fireplace," like cement and wood ashes. Our house has no fireplace. The chimney is in use only by the water heater vent, and it has a cap to keep rain out. We'll go for days without smelling this smell, and then it will suddenly permeate the living area (on the first floor) without any rain in the forecast, without the HVAC being on. It's as if a fireplace has been secretly hiding behind our sofa, and it was unable to suppress a fart. We are at a loss to explain it. Any ideas?