This is amazing. I have never even looked this up anywhere, and I just saw the first post while looking for something else.
About 10 years ago, I started taking a medication every day, and not long after, I began to smell -- smoke, dry metal (as in a machine shop), dust, a barn hay dust, dry (it is so dry a smell I want to say I am smelling dryness) -- any and all of those wrapped into one odor.
I asked my doctor about it as a side effect from the medication and he said it was not known to be such. Nevertheless, I switched medications and it went away. I resumed the first one, and it returned, but less often. I continue to take the medication that seemed not to induce the smell, but even so, every now and then, it returns. Thisls me it is not due to medication, and was probably a coincidence.
I have called it an olfactory hallucination, and I think it's more or less accurate. In other words, not resulting from anything external -- like a real environmental odor, and not because of a physical ailment somewhere else in my body. In fact, by calling it an hallucination, I am theorizing it as a neurological phenomenon.
I had not considered having it "checked" -- how would that be done, anyway? But reading all these other accounts makes me think it is, while not common, certainly not my singular experience. And, some posts here indicate that not everyone's experience is the same. I know no one else can smell what I smell no matter how close they are to my nose.
I do have some neurological quirks and I wonder if this has anything to do with them, or whether the neurological system, having a vulnerability, might easily be disposed to another.
A mystery. And, I am glad it is not as frequent and intense as it used to be. It was a constant distraction, was very unpleasant.