I experienced this for the first time
about 6 years ago. A day
or two before it happened, I was burning about 50 copies of
a CD, which does cause a slight burning smell in the house, so I wondered if this triggered it. It lasted about 5 days.
Since then, it's happened 4 or 5 times, each time lasting for a few days. The latest time was 3 days ago - I was in the car with my husband, there was a bus idling in front of us, and I smelled exhaust.