fishing supply box's dig spot is screwed up somehow. can dig it up fine but the metal detector doesnt report it as being in the ground, more like some point above the dig spot.
Viewing post in Voices of the Void 0.8.1 Bug Reports
The exact spot also moved to 176:-460, I checked with the map right after I fished it out. Or maybe it's impossible to dig it out if you only know the coordinates and have to point the metal detector at just the right spot for the box to actually spawn and become available to be digged out, at least that's how I managed to get mine.