I've read through the previous posts on this topic and haven't found anything that quite matches my situation. Here are the details...

Period in question: 8/5-8/12/2012
Weather during period in question: hot and humid, some rain, some sun,light breezes or still
Bathroom: powder room with sink, located on middle floor. We have a basement below, second story above.
House: built in mid-1970s
Sewer/Septic: Sewer
History: We've lived here 24 years, replaced all toilets 1 year ago, everything OK until this past week.

The odor smells like a chemical toilet, rather than sulfur or raw sewage. Seems to wax and wane: at its strongest, I could smell it from the top floor of the house; at its weakest, it was barely detectable.

I'm baffled because the toilet and sink are used daily, so the trap should have water in it. The toilets were replaced a year ago, so I'd expect any issues (such as a punctured sewer line) to manifest themselves before this.

I'd appreciate any help.