Fermi's paradox demonstrates it's almost certainly not a light sail. For it to be a light sail which was observed by chance by our totally unprepared observatories after a window of less than a century but also consistent with all our observations of the universe, we would need to hypothesize that we got extraordinarily lucky to observe a super-rare artifact, or that countless (or one vast) interstellar, pan-galactic civilization throwing out light sails in every direction constantly which also collectively choosing for millions/billions of years to avoid any kind of stellar engineering or direct colonization or Dyson sphere-like artifacts or even atmosphere composition tweaks visible on exoplanet spectrographs all carefully preserving the illusion of a 100% dead cosmos in every inch of our Hubble volume, or that alternately, interstellar civilizations are super common and in fact routinely throw out solar sails all the time, but some sort of interstellar-capable Great Filter kills them almost instantly before they can have any visible effects, leaving just a bunch of spreading artifacts. (And this Great Filter can't be any of the usual ones like asteroids or nukes or pandemic or development of intelligence.)