I had similar surface rust issues in the trunk floor of a 70 Newport, 35 years ago. Humidity. Age. It just happens.
Impster is a weirdo. No cracks or faded areas inside (save the T/T steering wheel), but rust spots here and there. It´s hard to think that the weather of Finland would have done it all in about 20 years after being in CA before.
But then dad has his 79 LeBaron, which was imported when new and it had badly faded parts inside, but not very much corrosion.