Carbon Monoxide

It's a well known fact that carbon monoxide is colorless, tasteless and odorless, as well as deadly. This lack of features is precisely what makes it so dangerous... there is no way to sense the slow seeping of carbon monoxide without the modern technology of a CO detector (or, failing that, the proverbial coal mine canary.)

What you might not have known is that when a carbon monoxide detector goes off in your house, while you can't smell the carbon monoxide, you will immediately smell the scent of your own fear. What does it smell like? As the alarm rings in your ears, you would swear that the pungent odor of your fear is exactly like the odor carbon monoxide would have, if it had any at all.