I've had phantom smells most of my life. Sometimes I'll catch a whiff of something very specific (the extremely expensive specialty detergent that my friend's mom used in middle school), and sometimes it's pretty general (bacon, floral smells). Sometimes other people can smell it too, which is kind of weird.
There was one day at my last job where I was walking between the feront door and the area I worked, I had just let a customer out (our area opened before the rest of the store). I had to pull someone else over to validate what I was smelling. There was about a two foot square area that smelled, inexplicably, like hardboiled eggs. There was NO reason for it, it hadn't smelled like that five minutes beforehand when I walked the last customer out, and it was ONLY in that tiny area.
I've also had pipe smoke follow me around. Specifically, chocolate flavoted tobacco as smoked through a pipe. The only person I've ever met who smokes a pipe is still alive and well, and not really stalking me, and this happens in places where smoking isn't allowed and I'd be able to see if someone was smoking a pipe near me. It's a very specific type of flavored tobacco, too, so I just don't get it.