filing, sanding, etc., will probably remove the ability to tell if it is a crack.
(this is what caused at least one turboprop airplane to crash due to propeller blade separation; the servicers "polished" the insides of the blades, unknowingly eliminating the evidence of the fractures (from corrosion) that were propagating thru them; some info here:
https://en.wikipedia.org/wiki/Atlantic_Southeast_Airlines_Flight_529
but no real details of the specifics; couldn't find those in a minute's search).
Instead, first try getting better more extreme closeups in very good light (direct sunlight often helps), with different angles. You may be able to tell if it is a raised bit of debris, or a recessed crack, or just a chip in the paint, etc.