You can actually smell fear because aromas bring back memories due to sense being more active when we are frightened

A scientific study from late 2013 suggests that people can indeed smell fear since our sensory cells learn to link certain aromas to it. This means that you can feel scared simply by smelling something despite the fact that the brain may not have registered the reason you should be scared.

A team of scientists from Rutgers University in New Jersey established that our sense of smell becomes more active when we’re scared. As such, the smell you happen to detect when you’re frightened is ingrained in your memory, and the fear will be evoked the next time you sense the same smell: smelling fear quite literally. Experts say this is due to the fact that the nerve tasked with transmitting messages from the nostrils to the brain, the olfactory nerve, is located right next to the areas associated with emotional memory.
This is the same rationale that conjures the memory of someone you knew, or know, whenever you smell a certain perfume, for instance. It’s a finding scientists believe could be used to treat post-traumatic stress disorder where attacks are often elicited by senses.