Lot of information to catch for me... thanks for your frequent replies.
Made this finding last night on my own. It also creates this visible beam if you remove the first window completly.The half window beam happens where the rays coming from the sun through the window hit the second windows inside the room.
Sorry for the troubles the blend file caused. I'm afraid all the adding and removing of faces damaged the data quality. I'll set it up new.Btw., in your new file, the interior windows were not fixed correctly, your old glass-to-dust plane does not overlap with the remaining glas-box. Also, I would run a "remove doubles" after joining to really fuse them back into one object.
I have already realized that this volume feature is missing for all other light sources than mesh-lights.IIRC with Bidir you currently have the problem that you can't define a starting volume for light sources (apart from meshlights).
So you have to resort to tricks like boxes in some cases because of this oversight/missing feature.
Could we integrate such a feature in upcoming versions, please?
Does it also infect the handling of the "environment" light sources?
I thought Sky and Sun and HDRI rays does always start in "world volume". Further I thought that the detected rays have to be in the camera volume. While having those two information, an unique solution of this volume problem should be possible (at least for this example).
Do you mean that a path engine may understand the setup correctly? I'll give it a try.For the unidirectional engines this doesn't matter because all rays start from the camera there.
In general, is there something wrong with my way of thinking or did we found kind of a bug?