I've experienced phantom smells from time to time, but they just go away by themselves. The longest I can ever remember having one, was probably just a few hours.

I don't know if this is relative or not, but with me, it is always the smell of something burning - anything from wood to electrical - and always in or around the house. I drive myself bonkers looking for the source if I am home by myself, but if others are home and say they don't smell anything, then I just ignore it.