When faces are coplanar or almost coplanar due to some unit (or scene scale) issues you can see some artifacts like this. I’m not sure if this is the case for your current scene, but your old scene seems so.
Assume you have a box with zero height. Then the bottom and top surfaces of the box are coplanar and on such cases menral ray creates unwanted textures or shadows on the faces.
This may happen if two surfaces are close enough and the scene is very large compared to the distance between them.
Bias parameters on shadow settings may help reduce unwanted shadows sometimes, by simply moving the shadow away from the shadow casting surface.
CHECK THE MATERIALS:
As a recommendation for your current scene, consider assigning the same material to all the objects on your scene. (preferably, one that mental ray recognizes- like arch&design mat.) Let’s see if it’s a material problem.
I have to admit that mental ray does create such artifacts for no reason at all.
CHECK TO SEE IF THERE’S SOMETHING HIDING BEHIND:
Try to eliminate possible factors by simply selecting your ground object and the problematic proxy building objects, then pressing Alt+Q. This hides all objects other than the selected. We may sometimes duplicate objects unintentionally. Then try re-rendering.
IF IT STILL EXISTS:
Assign a stl-check modifier to see if there are any errors like open faces or double faces on the mesh. Ground object may not be solid (one with thickness) so it will also be reported as errors.
Consider adding it some thickness by shell modifier. Also try the cap holes modifier. It may not help correct a bad mesh but at least, may cap holes and your problems may suddenly dissappear. You may then find out there is a problem with the mesh.
A DUMB BUT POWERFUL RECOMMENDATION:
Change the renderer to scanline, and then back to mental ray. Believe me, it helps, by resetting some parameters and may solve many of your problems, even improving rendering times.
Let’s see if any of these work.