If you suddenly have started smelling things sometimes that are not there you could have Sinusitis.
Sinusitis is a condition where the cavities around the nasal passages become inflamed.
This causes you to smell things that are not there and usually it will go away on it's own within a few days to a week but if not you should see a doctor about it.
Other conditions that can trigger phantom smells and cause you to sometimes smell things that are not there are things such as temporal lobe seizures, epilepsy, or head trauma.
When that happens you'll sometimes smell a chemical smell or burning like smell that is not even there.
Neurological disorders can also cause you to smell things that are not actually there around you.
That can be caused by problems with your nervous system.