Error Coloring & BSP?

I am receiving three Lightmap UV errors during the build process, it recommends to ‘Enable Error Coloring’ which I have done, what should I expect to see on the map to track down these errors?

The map is almost entirely BSP, does Error Coloring work on BSP or just static meshes?

The map is a 32 story building and I think I have a few duplicate brushes on top of one another but with 7500 brushes, its hard to find. :frowning:

I am aware that BSP is probably NOT the way to construct a map of this type nowadays, the UE4 tutorials were a tad deceiving in this regard when compared to comments/suggestions Ive come across on the forums, answer hub etc.

Thanks for any suggestions or pointers anyone is able to provide.