We have a Fujifilm X1 camera whose lens developed similar patterns. When we took it to a service centre, we were told that it was fungal growth, so I am making an educated guess that this could be the same issue.
High humidity creates an ideal environment for fungus to grow. It happens quite often in camera equipment, although it is usually more visible on a lens.
The image below shows fungal growth on a camera lens, and the pattern looks similar.
The marks on the metal frames in your other images also appear consistent with this possibility.
I do not want to cause unnecessary concern, and I may be mistaken, but I think it would be advisable to have the sensor inspected thoroughly by a qualified technician.