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Constant or occasional feeling of smelling something in the absence of any smells needs to be investigated. Olfactory symptoms can happen in certain ENT disorders with postnasal catarrh. Olfactory hallucinations do also happen in some psychiatric conditions. They may also be associated with auditory (hearing) and visual hallucinations.
Naturally, this will require a thorough physical examination, ENT examination. Based on the outcome of ENT examination, this may need further consultation and investigation. Thank you.