Phantosmia is an olfactory hallucination: perception of
smell without an odor present. These imaginary smells can range from unpleasant to enjoyable. Causes include brain injury or seizures in the temporal lobe, various neurological disorders, overactive olfactory receptor neurons (ORNs), or the loss of inhibitory neurons brought on by a sinus infection or some other type of head cold.
I had a bout of phantosmia, in my case it was an electrical burning type smell, that lasted for a few days.