Originally Posted by wiki

Phantosmia is the phenomenon of smelling odors that aren't really present. The most common odors are unpleasant smells such as rotting flesh, vomit, urine, feces, smoke, or others. Phantosmia often results from damage to the nervous tissue in the olfactory system. The damage can be caused by viral infection, brain tumor, trauma, surgery, and possibly exposure to toxins or drugs.[8] Phantosmia can also be induced by epilepsy affecting the olfactory cortex and is also thought to possibly have psychiatric origins.[citation needed] Phantosmia is different from parosmia, in which a smell is actually present, but perceived differently from its usual smell.

Olfactory hallucinations have also been reported in migraine, although the frequency of such hallucinations is unclear