Plus if it was double sided (or any of a number of other likely causes), I'd expect to see it with other MAT files.  That's what has really got me stumped, why only this one MAT... AND... why did it suddenly appear when the only change I originally made was a slight change to the scaling of the object mesh when exporting?   Just doesn't make any sense to me.  There's nothing unusual about the mesh in that area, mostly quads and a couple of fans of tris under the curve at the top.  Pretty simple mesh in that area (whole wall is only 1800 polys, mostly quads, a few fans of tris and no ngons).

It probably won't matter to the final project, may never appear again.  But... okay so I'm obsessive.  LOL

Short of giving out the object mesh to other to experiment with, I'm not sure what else to try.