Hiya UHF! Your mirror looks like it has general noise around the left and bottom (most perceptible anyway). Quick questions: is the surface one that was converted from the original mesh, and if so, is the mattress intersecting it? 

For the first part, check the surface of your mirror in studio in wireframe mode and see what kind of construction the part of the mirror looks like. It could be that the mesh has warps and minute imperfections. In that event, placing a primitive rectange over it scaled to size inside the frame will give you a clean, smooth surface to convert your mirror. Also, if the mattress mesh is intersecting your original model, that can do funny things in Lux anyway. 

I haven't used the actual mirror surface in Lux in awhile, but for expermienting purposes, you could try setting it to metal and using the chrome preset maxed out to 10000 on the polish setting. If your mesh is warped, you'll definitely see it with that! (funhouse mirror style) 

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