Parosmia and Phantosmia: "Phantosmia involves olfactory hallucinations -- that is, there is no odorant present, but the affected person reports smelling something, usually something unpleasant."
The strangest phenomenon. I've been experiencing it fairly often lately. I'm sitting here at work, and there's a vague smell in my nose like I've been smelling chlorine, like at the pool. The other night it was foods, and the night before that, it was an oily smell, like that VO5 hair treatment in a tube. I wish I could explain it, but it seems I'm going to have to do more research. The only problem with that is once you start reading those sorts of things, it would be easy to be convinced you have all those ailments.