Hi Pete,
It projection of Vox details to Retopology. If your experiencing strange normals are shading after merging it more than likely the global state of the voxel volumes your trying to project details from.
I currently have a checklist of things to check b4 I merge to paint. First thing i check is that all volumes are set to global space. Its important to do so not only for projecting the details at merging time but say i want to merge different volume into a single volume, sometimes problems can occur when merging volumes together if they are not set toglobal space.
Excuse my bad grammer its late, and i had a beer.
LJB's post in set visible vox layers to global space was marked as the answer