My guess is they're caused by the venetian blinds.

Link to original image 12.6 Mb
Lighting in SH3D
- 2 halogen lights 100x100 cms, at 20% (at left edge, outside of image)
Render with PVR
- settings = Q4, path, 2 bounces, algorithm=fast
- ceiling lights = NO, enhance external brightness = YES
- original size= 3000x2000 pix, time +/-3 hrs @ 6 cores

