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What your mother is suffering from is probably olfactory hallucination or phantosmia. It is usually due to seizure, or due to schizophrenia or depression or due to an aura of migraine. It can also be due to a problem in olfactory receptor mechanism. This could be due to a smell getting trapped by nasal polyps or tumors. Since it can be something serious, your mother should be seen by a psychiatrist, ENT specialist and a neurologist. It is difficult to comment beyond this at this stage.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!