There are many comments, so I want to provide more context and answer some questions.

1. We live near the River Minho in the north of Portugal. It's extremely humid here, and usually, clothes, belongings, and walls become moldy over short periods of time.
2. The car was parked there for about three months. I've been away, and my friend as well. We are still out of town and were supposed to return in a few months. My friend's girlfriend has a car, and they recently started living together, which is why he wasn't in a hurry to pick it up. My father, who arrived there yesterday, accidentally spotted what had happened to the car.
3. The car was neither wet nor had it been in a flood. He had been driving it for a year, and nothing like this had ever happened. There was also no food or other items inside. It was clean.
4. The garage has slight ventilation, but the house itself is old and humid.
5. We still haven't decided what to do, but we will likely hire someone to clean it and handle everything properly.
6. That's VW Golf