I would stick a big fin in it , put the board in the sun for five minutes then spray area with soapy water while pushing the fin HARD sideways . If there's no bubbles, leave or bog it . It's probably filler just chipped off .
I owned a 2010 Fanatic Hawk that did that. I did the repair and then I filed a very shallow chamfer around the fin box so the fin would not grip the top layer when removed.
