As mysteries go it's pretty prosaic, but for the past few years there has been the faint memory of a long-gone lunch that lingers around my desk at work with no reasonable explanation. Since I inherited my office several years ago, there has been the faint but unmistakable smell of tuna salad that emanates from the desk. I noticed it when I first moved in and put it down to an uncleaned morsel  or drop of fish oil that had been overlooked. But this ghost scent has persisted. The desk itself has been moved and scrubbed and wiped and endured pandemic-era disinfectants. But still, a couple of days ago it returned. It's not rancid or unpleasant or strong. I'd even say it smells kind of fresh. And it's not there all the time either. It seems to surface every few months without rhyme or reason  - on hot days or cold. Could it be an olfactory hallucination? If I were to imagine a scent that represents the weekday lunches of my lifetime, this would certainly qualify. Tuna sandwiches followed me throughout my elementary and high school years  - enveloped in tinfoil and traveling in plain brown paper bags like packages sent through the mail - up until my working life and the Tupperware salads, whose only concession to current sensibilities are the dolphin-friendly tins. I've given up trying to find the source of it and accept now that this persistence and one of my senses are in on some kind of private joke.