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I don't know for sure if that's the case but as was pointed out, properly reseating the toilet is such an cheap & easy thing to do that it's the best place to start. I was amazed that even though mine wasn't leaking water the smell came from so many seemingly unrelated places.
I realize this thread is quite old but I am desperate. I have exactly the same problem in a home I recently purchased. In addition to the steps you have already completed, I had the septic line dug up and re-routed to make sure drop was adequate. As stated, only have the problem on windy days, have not noted wind direction. Also installed one new toilet and had the other reseated and caulked the outer rims. Any assistance/suggestion would be greatly appreciated.
Sorry, I made the rookie mistake of not realizing there were other replies, so missed a page where the answer may lie. I believe both my baths were tiled well after the house was built so the wax ring, the most obvious culprit, which I had eliminated because because of having it reseated, may be the problem after all. Will re-seat myself so that I can determine whether or not a good seal is being made. Thanks Guys!