Olfactory Hallucination: This could be due to various factors such as stress, anxiety, or other psychological conditions. The brain can create the sensation of smelling something that isn't there, which in this case is cigarette smoke.

Exposure to Secondhand Smoke: Continuous exposure to environments where others are smoking could lead to the persistent smell of cigarette smoke.
Residual Smoke from Previous Exposure: If the individual was recently in an environment heavily contaminated with cigarette smoke, the smell could linger on their clothes, hair, or in their nostrils.
Nasal or Sinus Infection: Certain infections can alter one's sense of smell, potentially making them more sensitive to certain odors or causing them to perceive smells that are not present.

Temporal Lobe Seizure: Although rare, seizures in the temporal lobe can cause olfactory hallucinations. Missing this diagnosis could lead to untreated seizures and potentially severe consequences.
Brain Tumor: A tumor pressing on areas of the brain responsible for processing smells could cause persistent olfactory hallucinations. Early detection is crucial for effective treatment.
Neurodegenerative Diseases (e.g., Parkinson's Disease, Alzheimer's Disease): Some neurodegenerative diseases can affect the sense of smell early in their progression. While smelling cigarette smoke all the time is not a common presenting symptom, changes in olfaction can be an early sign.

Phantosmia: A condition characterized by smelling odors that aren't actually present, which can be caused by a variety of factors including head injuries, viral infections, or exposure to certain chemicals.
Hyperosmia: An increased sensitivity to smells, which could make the smell of cigarette smoke seem more pronounced or persistent than it actually is.
Charles Bonnet Syndrome: Typically associated with visual hallucinations in visually impaired individuals, but in rare cases, it could involve other senses, including smell.