Mystery Windows in Paris ...

... they're in the Butte aux Cailles area of Paris's 13th arrondissement.

This mildly hilly sector is named after Pierre Caille, who acquired it in the 16th century. A number of windmills were built on it, presumably drawing from the Bièvre river (long since underground, I believe).

I'll confess that I've boosted the saturation and contrast considerably in this image. The windows are obviously primarily reflecting the building directly opposite (behind me the photographer). But I'm not sure about the two sets of diagonals in the right-center--they may be internal staircases, behind what seem to be curtains.

It's worth a large view.