Thanks Alastair - looks like two possabilities:
1) only the tracking has failed, and PassAt is still on correct course
2) anything from total systems failure to boat sinking
Maybe, if you put out the word to the right guys ( mostly military ? )
Someone may get some cudos by providing a photo and location.
A French Air Force recce aircraft did it many years ago for the French boat.
Obviously examples for Snoopy including sailors passing,
and for Royal Marines - but that extended to physical pranks & favours ;-)
I am very familiar with the friendly rivalry between military - despite their Governments ;-)
Those predictions of prevailing winds might give a clue as to the "best guess" on location, if still floating.
Good Luck ! Take Care - and don't get caught ;-)
Robin & Snoopy