Looks like it was Struck Through Grease/Debris. I'm not seeing any displaced or raised areas of metal adjacent to the incuse area.
I have a pair of 1992 Silver Eagle coins with matching Strike Through areas in the field that look a little like this coin. Take a look if you'd like.
http://goccf.com/t/358464
ANA ID: 3203813 - CONECA ID: N-5637 Clean a coin that may be worth collecting? Please DON'T! When in doubt, leave it dirty!!
