I have seen in first/business class people with shorts, sandals, T-shirt, and a blazer! They often travel with a tennis racket even in the middle of the winter. It makes sense as otherwise you have to squeeze in your blazer/jacket in to an over night bag. Why not wear them regardless of whatelse you are wearing.
How about pajamas? I have seen passengers changing into night clothes on long distance flights and then wandering all around the plane. I believe some airlines even give first class passengers night clothes.
I have seen a few who would cover themselves completely by a blanket, even covering their heads, and look like an apparition, deeply asleep. They often over sleep and are the last one to get off the plane. So you would never know their identity. I suspect they are celebrities.