When reading this, you should generally be prepared that length of it may be smaller than all sectors of your scene. That's because sometimes we don't initialize VisibleSectors fully.
In this case you should assume that not initialized sector indexes are visible. And always you can assume that Always Sectors[I].VisibleSectors[I] = True, i.e. the sector is visible from itself (assuming that VisibleSectors has enough length to contain I).