I believe it is a combination of several of the explantions above as I have examined similar surface defects while working with microscopes, macroscopes while trying to determine contributing causes of various metal defects in drawn parts in the stamping industry. 1st in "this case" the metal is stamped & movement occurs - possible and most likely - die/slug movement, 2nd and always present "metal flow". This flow more pronounced at or near the lettering cuz there's a void or a positive - (the letters themselves) in the dies will fill with migrated material from an adjuscent site. As the metal moves from one place to another it leaves a telltale pattern on a molecular level which we can study with chemical etching, or coating with something that highlites the highs and lows of these surfaces to make the "pattern/defect and in our case ghost letters" visable to us.. in this situation it's just plain easy to see and chemical etching or magnification are not needed.