I honestly think it was a balloon.. Hear me out.. A huge, foil fabric balloon. When they're partially deflated, they flatten out... looking disc'ish. They get blown around erratically, as their shape morphs from the twisting and turning. General population had never seen this material before, so when 'regular' folk, even lower military, came upon the wreck, they though they had interstellar material. Seems more plausible than other things I've heard...