Phantom odor perception

A new study revealed  1 in 15 Americans (or 6.5 percent) over the age of 40 experiences phantom odors, which is smelling an odor that is not actually there. 

Phantosmia (phantom smell), also called an olfactory hallucination, is smelling an odor that is not actually there. Phantosmia can occur in one nostril or both. Unpleasant phantosmia, cacosmia, is more common and is often described as smelling something that is burned, foul, spoiled, or rotten

Ref:
https://www.nih.gov/news-events/news-releases/stinks-1-15-americans-smell-odors-arent-there