The flash would prove that proton decay really happens. The flash would mean that the matter of the proton - the solid stuff - had turned into the energy of the flash (E-mc2). Totally. Nothing left behind. No ash. No smoke. No smell. Nada. One moment it's there, the next moment - pffft - gone. What would it mean? Only this: Nothing lasts. Nothing. Because everything that exists is made of protons.