Smudge looking marks onscreen

If you suddenly noticed it, this is most likely caused by a large piece of dust on the lens. It is hard to clean, you would need to take the whole TV apart in order to get to the lens (mounted on the optical block).

It could also be caused by a failure of the optical block, which had inherent issues. There is a pending class action law suit against Sony for problems with optical block that include smudges, hazes, and tints that appear after a certain number of hours.

Other causes could be a dirty portion of the mirror or a smudge on the inside part of the screen but these are rare because dust and contaminants do not usually accumulate on these parts of the TV.