Hi. I recently purchased a wusthof knife and today lent it to my mum for her to use it for the day. When she returned it I was shocked to find that there was a 1 inch, clearly visible scratch mark on the left side of the blade. At first I thought it was food stain so i washed it with hot soapy water and dried it, but it was still there. I was very disappointed because 1) mum didn't take good care of it and 2) clearly i expected wusthof blades to handle more tear & wear. Then just a few minutes ago I was chopping some veges and when I washed it with hot water and detergent i found that the scratch had gone! What could be a reasonable explanation for this?