There is some problem with VLM Vol.LightMap for dynamic shadowing.
Vol Cells are placed ( i suppose ) starting from 0,0,0 where is placed the centre of the cell.
So - in many cases - when our scene begins at Z=0 this first cell row (z=0) is taking lights for computing form interior and exterior. In causes that his Z0 Cell gives additional bad light at the bottom of the scene. Solution that I use now is add "envelope - exterior walls" walls that makes some border between exterior and interior walls (surrounds interior). In this space bewteen is darkness so with small VLM Cells (i.e. with size of 16 ) the dynamic light is ok. But it consumes more memory.
I have two ideas how to improve whole VLM Vol LM system:
1) Add offset to Cells Grid to have ability to move all grid in demanded direction to avoid that some cells are taking unexpected light from exterior
2) Develop VLM Importance Volume system ( like for Lightmass importance volume ) to set the VLM grid locally with demanded size ( i.e. scene has size 200 , but local VLM grid that surrounds i.e whole interior has 16 for better dynamic shadow resolution ) - this gives

This image shows the problem:

Click image for larger version

Name:	Schowek-1.jpg
Views:	18
Size:	127.8 KB
ID:	1376588 fantastic management for scene dynamic shadowing.