My Hornby Q1has been stored in a cupboard for a few years. It's nice and warm and dry in there. The model is wrapped as it came from the factory.
Taking it out for some photographs, I noticed that the sides of the smokebox were oily. Blobs of the stuff. Checking the piece of thin plastic wrapped around this area, there was oil on there too.
My best guess is that the lubricant from the chassis has slowly run into the plastic and then on the body of the loco. I can't remember is the loco was upside down in it's box but I suspect so.
Lengthy storage allowed this to happen. Fortunately, a gentle rub with some absorbent kitchen towel cleaned things up.
The moral of the story is run your locomotives - don't leave them in the cupboard!