I have had this experience of smelling things that are most likely not there. Often it is cigarette smoke, or perfume. Since nobody smokes in my household, and I don't wear perfume, it is puzzling. It doesn't happen in our new place as much, but I often wonder if it was the neighbors when we lived in our old place. Sometimes I will smell vague odors that remind me of people or experiences, and now I am inclined to believe it is related to memory.