Thanks for your replies ondebanks and Sergei. Sadly I don't have the same light conditions here in Berlin as when I came to that "reflections-conclusion". Most of the time it's cloudy and there's not enough light to cause such reflections at all.
However, I realized that it must have something to do with those reflections mentioned above when I tried to shoot in comparable light conditions. I stood app. 12ft away from the four windows of my living room and did some shots. The foreground was about 4 stops underexposed and when I pulled shadows, I had purple areas in the exact shape of my windows in the dark areas. I don't think this is coincidence.
IMO there is an explanation why other backs utilizing the same Dalsa sensor don't show these areas:
They don't have a red filter in front of the sensor.
They have more than 12 bpc color information (which might be enough to realize that a certain area is not red but that's just guessing).
may I ask what kind of system do you use? Maybe your lenses don't tend to show "lens flare" (due to better coating?).