The musk in your car can come from anything, and doesn’t always have to be whatever your kid spilled in the back seat last week, it can be a growing odor that builds and builds after several weeks. The most important thing is to find the source of the stench and clean it as soon as possible, don’t leave it for the next day. If it works its way into your carpeting, you could pay hundreds having it torn out and replaced because it has gotten under the upholstery and seeped somewhere that cannot be cleaned. Milk in particular can seep down to the floor boards, and only a professional can clean it up. Keep a towel handy just in case there is a spill so you can dry it up as soon as possible.
One story tells about a woman who left a whole chicken in the trunk of her car, and it was left to bake there for a week while she was away. She had to have the carpeting removed to get to where it had leaked. Often the smell is caused by bacteria, from food or other organic items that can be rotting. Some are simply obvious, such as cigarette musk that can work its way into your upholstery and never really smell clean.