The blast of horrid stink or the entrancing and mysterious smell that accompanies someone as they walk or run past. This is the russian roullette of smells and is normally unavoidable in close social spaces unless one gets really good at holding one's breath just to avoid how smelly that sweaty yoga instructor must've been.
John: Sweet weepin' Jeebus
, Karla needs a serious power-washing today. She walked by me near the copier and the doppler smell was so rank it stripped years from my life!