Its technically called 'phantosmia'. Most common cause or known issue that is related to phantosmia would be epilepsy. Mostt commonly temporal lobe seizures. There are many forms and a wide range of symptomatology to epilepsy. Although the common belief is that epilepsy is a "drop to the floor fit" it is in truth a misfiring of the electric system of the brain. Many people have petit mal or absence epilepsy (Meaning tiny or small seizure) As the secondary name implies "absence" is the usual symptom - where a person "blanks out" for a few seconds. Most often the patient is not aware that they have blanked out - observers may see a person having an absence seizure stop in the middle of a sentence and "freeze-up" for a few seconds, then suddenly they unfreeze and continue the sentence as if there was no interruption. This can lead to many 'interesting' side effects, from auras (which can be sight, hearing, smells, sensations) to the feeling of de-ja-vu. Behavioral changes during and after petit mal is possible as well. Most times it is something like hallucinations where spurn a neurological study that uncovers epilepsy. Other causes are usually brain based, brain injury, tumors, etc. Lastly and less frequently psychiatric illness, such as depression or schizophrenia, Alzheimer's. Rarer still it can be an onset symptom of migraine headache(s).

Many neurological conditions can cause this...tumors, epilepsy etc. You'd be best to get an evaluation from a neurologist.