As a car owner, this is definitely one of the weirdest things I've experienced in my life...
I took my car into the mechanic for a regular oil change. They call me a little while later and ask if I've smelled anything weird in my car. Nope. I have not. Well, they couldn't work under the hood because it smelled like vomit!
I have gingko trees near my car that drop those berries that end up smelling like vomit -- because they contain butyric acid -- the same chemical that dominates the smell of rancid butter and vomit, as Arnold Arboretum Director Ned Friedman explained.
I figured some of the berries had fallen into my car. Nope, look at the picture below. Some animal(s) has been taking the berries under my hood, eating them, and leaving them. Animals -- mice, rats, cats, wolverines (no, not wolverines) -- get under vehicles' hoods during the winter to stay warm. And apparently they bring snacks.
The mechanic had to steam wash the engine to get rid of the berries and the smell -- and I've got to bring the car back for them to change the oil after the smell has dissipated.