ghost whiff

To get a brief whiff of something that was never there.

I was sitting at my desk and I could swear that I smelled french fries -- must have been a ghost whiff.

Me: "I smell french fries, who's got 'em?"
Co-Worker: "Nah, no fries here man."
Me: "Really?!"
Co-Worker: "Yup. Really. Must have been a ghost whiff".