It is the curved top of a microwave oven out of focus against a white wall.
It looks like Demosaicing artifact. Have a look at
Hmm, that uninterpolated (and interpolated) “artifact” looks somewhat familiar. I have an old manual focus Distagon 2.8/28mm lens that produced a similar artifact when photographing sharply defined diagonal lines (in particular a test shot that included some diagonally-placed color post-it notes) when used with a Canon 400d digital camera, which had an 8MB sensor.
As best I could determine by perusing the internet at the time (perhaps in 2008), the problem might have been that the lens was “outresolving the sensor”. But I have no idea whether this really was the issue. And I haven’t photographed a similar sharp diagonal line with this same lens on my Sony A7, to see what happens with a 24MB sensor. And given all the shenanigans Sony A7 plays with their asinine (am I allowed to use this language on pixls.us?) lossy-compressed raw files, I don’t think such tests have much validity for the Sony A7.
Well, it is with a discreet dose of shame that I just discovered what the top of the microwave really looks like. Thank you for your answers, I am sure I can do much better than this
Thank you all!
Ha ha, it is a feature!