Sometimes, you feel like something is wrong. Most of the time, it’s just a feeling; it doesn’t mean anything. But, every now and then, something is wrong, you really are in danger, you really should be afraid.
Here are some articles on how humans can react to the unconsciously sensed smell of fear in the perspiration of others.
Here’s a paper on the human reaction to putrescine, a chemical associated with death and decay. Even when we don’t consciously notice the smell, we’re instinctively made nervous by its presence.
Here’s a talk by a woman who felt unexplained feelings of dread within her new home. At first, she thought it was a ghost, but eventually she discovered it was something far more deadly.