Do you live in an apartment or have close neighbors? My last pregnancy we lived in an apartment and the lady below us smoked, the smoke came up to our apartment through the pipes. AND people smoked on the sidewalk pretty close to our bedroom window! The landlord always said "no way, i know that lady and she doesnt smoke! "
Low and behold, once we moved out and i went to get my deposit back, she told me she kicked that lady in the apartment below us out for smoking!
So yeah i smelt it too mysteriously last time but we figured it out