I saw this in CSI: New York, so take this with reservation, although these shows don't always suggest new things that aren't real: They mentioned a substance that was tested on waste collectors. The substance itself didn't smell, but when applied under the nose released a chemical that made a stinky sulphur substance smell like vanilla. Although I am not sure if this would count for a smell illusion - more like a smell distraction.
Or smell disguise. Though one wonders if what happens is a new chemical forms that 'really' smells like vanilla. So it in itself is not an olfactory illusion: something smells like vanilla. However that's being fussy about an interesting story.
It is said CO2 doesn't have a smell. But when it reaches our mucuous membranes in the mouth and nose as we breathe it in, it reacts with the water in them, and forms an acid that we perceive has a foul smell. I suppose we'd have to go into the whole principle of smell, and when we can rightfully say that a substance has a smell, and when it doesn't. I suppose cocaine has no smell for humans, but it sure has a smell for dogs, for example.
