My father developed a sensitivity to mold, and he could walk in a building and smell a mold problem, even if no one else could. I saw it in person when we walked into a building and said he smelled mold. A month later, the building owners found a lot of mold in an outside wall due to a roof leak.
Perhaps you have developed a sensitivity to something, which can be a good thing if you can find the source and eliminate it. You may also want to have your heat and a/c system checked for mold.
Maybe you are just sensitive to this smell; maybe you have some kind of very keen sense of smell that allows you to smell more than most. Females generally have a better sense of smell than males, if that helps at all.
I'd preferably chock this up to scent sensitivity before suggesting it might be an olfactory hallucination or something with more serious implications. If it were a hallucination, it could mean much more dire things, and you seem to notice its source as being your car, so that's a good thing.
Do you spend a lot of time in your vehicle? It's possible that the scent lingers in your nose... and if the scent is unpleasant, you're not very likely to habituate to it and forget it's there. You might just be more sensitive to this scent compared to other members of your household. Maybe try air freshener in your vehicle and see if this lingering scent subsides.
You may have a sinus infection. That is one reason you may smell things that no one else can, or even a neurological issue can lead to phantom smells. Best to see your Dr.