HI i might be out of my depth here but im sure i heard that if you take 2 pieces of metal and stroke them together in the same direction continualy for a long ..long time they eventualy become slightly magnetic[a labour intensive version of strokeing a piece of metal in 1 direction with a magnet ]..i have noticed this in old drillbits that develop magnetism from spinning continualy in 1 direction against metal.[i asume this is why they become magnetic]?
Perhaps smiths who use metal files could inadvertantly cause magnetism in blades from useing a file in this method, gradualy magnetiseing the file and then whatever blade he works on [this is relying on him haveing a fileing technique where he files only on 1 direction and lifting the file on the return stroke ]i see this fileing technique in woodwork but not sure about metalwork.
Also mabe this could be the case for a stone sharpeing/grinding wheel[if it had a slight metal content]?Also this might explain why some blades have diffrent strength magnetism on diffrent parts of the blade that required more fileing.
This may sound silly but i thaught id mention it