Yes, this guy was found in our cellar. According to wikipedia, they sometimes do live in basements and such. It is called Tigerschnegel, which means tiger slug. It eats mostly mould, dead small animals and egs of other slugs.
It is rare and protected. We released it in the next wood. I’m sure that change of scenery wasn’t really appreciated, but there was danger that someone steps on it in the cellar, and it could have eaten poisoned slugs in the garden.