Vinegar is supposed to remove that type of mark (even if it has been "permanently" set with heat). I would test it on some scrap fabric first, both with an unironed mark and an ironed mark. The mark that won't come out may have somehow become set by a heat source, although I can't think offhand what kind of heat source would hit just a small spot.
Edit: Nothing I know of works on pre-printed panels. Vinegar is supposed to work on the blue vanishing markers, though.