Your brain is able to literally smell the sense of fear. It’s not a sixth sense. Nothing paranormal or supernatural at all.
Instead, this unconscious ability can help explain, for example, why ghost believers, if being in the same allegedly haunted house, often trigger each other to share the same spooky experience.
As a matter of fact, the smell of fear is very contagious (cf. the mass hysteria phenomenon, in which a large group of people exhibit the same state of mental agitation).
In many animals this ability to smell the chemicals of fear is pretty advanced. Just think of dogs who nowadays are trained for detecting cancer, bomb chemicals, truffle, dope – you name it.
It’s all about the vomeronasal organ (VNO), an auxillary olfactory organ located in the brain. The VNO contains sensory neurons specialized to detect chemical stimuli. These neurons target the amygdala, the brain’s fear center.
The vomeronasal system is important for reproduction and social behavior (think of pheromones, chemical substances secreted externally in order to influence the physiology and behavior of others belonging to the same species).
The presence of a VNO structure in adult human beings is still debated, expecially how functional such a VNO in adult human beings might be. For details, have a look at this paper: http://chemse.oxfordjournals.org/content/26/4/433.full .
Here’s a quote from the abstract: [There are] conflicting evidence for and against human VNO function but chemical communication does appear to occur among humans. However, several examples reported in the literature do not meet the proposed definition for communication by pheromones: ‘chemical substances released by one member of a species as communication with another member, to their mutual benefit’.
And here are three more articles about this interesting topic: 1) http://www.telegraph.co.uk/news/science/3545435/The-smell-of-fear-is-real-claim-scientists.html (Title: The smell of fear is real, claim scientists); 2) http://www.abc.net.au/science/articles/2015/10/20/4333431.htm (Title: Sharing the scent of fear); and 3) http://www.livescience.com/24578-humans-smell-fear.html (Title: Humans smell fear, and it’s contagious).
Many woos are convinced that they can detect paranormal scents. For instance the scent of a ghost. Here is one of the best articles I’ve found on that matter: http://www.assap.ac.uk/newsite/articles/Paranormal%20smells.html (Title Paranormal smells). Highly recommendable reading!
Among the bizarre olfactory phenomena mentioned in that article are phantom smells, for example phantosmia. This is a form of olfactory hallucination, i.e. the perception of a smell in the absence of any physical odors.
If there is a misinterpretation of an existent physical stimulus,the proper medical term is parosmia. Such olfactory dysfunctions (a.k.a. dysomias) are characterized by the inability of the brain to properly identify an odor’s “natural” smell.
These distortions of smell are not as common as phantom smells, but parosmia is still something that must be taken into consideration whenever a certain odor is associated with a haunting.