Mucous threads normally appear in urine, and it is only a problem when a large amount of them are seen, which might indicate inflammation or infection of the urinary tract. Any abnormal results are "flagged," and reference intervals help the doctor determine if anything's out of range (such as color, appearance, specific gravity, pH, protein, glucose, urobilinogen, ketones, bilirubin, blood, nitrite, leukocyte, microscopic exam). "None seen" does not normally refer to "reference interval," but usually other abnorml findings like skin cells, white blood cells, red blood cells, bacteria. Anything abnormal would have been evaluated further, but since you were told thta your results were "good," there should be nothing to worry about here.
I hope this helps!