We had started a fire in the first floor fireplace our first cool day, and after a while, walked up to the second floor and noticed a bad sewer like smell. I smelled each drain in the two upstairs bathrooms and couldn’t determine a specific smell coming from the drains. I took and used the baking soda, vinegar, followed by hot water method to flush all the sinks and showers (which are all used daily). It seemed to help, but as soon as I ran water in the sink, I could smell it immediately. I made sure that all traps were refilled with water. I checked the sink smell out the next morning (after the fireplace fire was out) and the smell could not be created again with running water. I was able to recreate the same effect the next day. I have never noticed this problem at any other time. Any ideas what might be going on?