A patient repeatedly reports smelling burning rubber, on and off over several days, though no one else in the room notices anything and no source can be found on careful checking of the environment. What is this?

This is an olfactory hallucination: a smell perceived with no external source, here confirmed by others' inability to detect it and a careful check of the environment finding nothing — exactly the pattern the phenomenon's own text describes, including the classic unpleasant quality (burning). It is not olfactory reference syndrome, which is a delusional belief about emitting one's own bad odour, not a perceived external smell. It is not a genuine environmental odour, which the careful check rules out. And it is not an olfactory memory, since he is describing a present-moment perception, not a recollection he recognises as such.