Folks think of cell Rot as the typical lower line of corrosion across the blade however that is not always the case. When celluloid goes sometimes it starts as a pinpoint area on the scale and only affects a tiny area on the blade. I have a Revisor which upon inspection one day it had a rust stain on it in one small area near the toe of the blade. I didn't snap as to what was happening and just cleaned it up and reoiled it and put it back. I checked it a couple of months later and the stain was back accompanied by a small pit. The scales looked great and there was no indication there was an issue with them but there was a bad area right where the pit happened. The rest of the blade was pristine.

So, reoccurring rust rapidly worsening for no apparent reason is another symptom.