The sensation of smelling cigarette smoke when there is none is known as phantosmia, or olfactory hallucination. It can be caused by a variety of reasons, including:

• Migraine aura: About one-third of people with migraine aura experience phantom smells such as cigarette smoke just before or during a headache, lasting from minutes up to an hour.
• Nasal or sinus issues: Chronic sinusitis, nasal polyps, allergies, or sinus infections can distort smell perception and cause phantom smells.
• Residue of real smoke: Lingering cigarette particles on clothes, furniture, or air vents can make the brain "smell" smoke even when no one is actively smoking.
• Medication side effects: Some antibiotics and antidepressants list phantosmia as a rare side effect.
• Neurological causes: Rare but serious conditions like temporal lobe seizures, stroke affecting the olfactory cortex, brain tumors, or neurodegenerative diseases can cause phantosmia.
• Common illnesses: Colds, upper respiratory infections, dental issues, or exposure to irritants can trigger the sensation.
• Psychological factors: Stress, anxiety, obsessive-compulsive disorder, and other mental health conditions can sometimes cause phantom smells.

Phantosmia usually comes from problems in the nose or sinuses rather than the brain, but if the symptom is persistent or concerning, a medical evaluation is recommended to rule out serious causes.

Thus, smelling cigarette smoke when none is present can be due to phantosmia arising from migraine, sinus issues, residue of smoke, medication effects, neurological or psychological causes.