My theory of it landing on Cloudbase and then being whisked off to the South Pole is still in the running....
...only it seems that the Mysterons attacked Cloudbase and the 777 was pushed overboard in the ensuing skirmish. Possibly a result of that racist Colonel White and his refusal to listen to Capt Black. I reckon a couple of hours torturing Harmony with Piwi posts would break her into telling everyone the real truth....