Happened to me too. Never had scale mite in my flocks...ever. Or any other kind of parasite. Then I took over some "free" hens that a friend begged me to take from someone they knew. They looked good, no evidence of scale mites on their legs..but there was a red streak running up their legs that I had never seen before. Now that streak is very familiar to me..it is something that is left behind after a scale mite infestation on some birds. My flock got their little surprises and I was dealing with SLM for the first time. Enter the NS after trying the Vaseline, cooking oil, olive oil, bag balm, etc.
NS had it gone in one treatment and the residue from their feet that was left on the roosts seemed to prevent recurrence.
Never again on the adopted birds, "free" birds.
Not worth it.