For about a month, we'd noticed a malodorous presence in the car. It smelled like sour milk. We'd pinpointed one of the carseats as the culprit and had washed the seat inserts. That helped, but the smell was still there.
Today, I went to wash the seat cushion again and found in deep in the abyss of the plastic moulding frame: a piece of cheese. I'm not sure how long it had been there, but it's not there anymore. Such is the sweet non-smell of success!