In 1822, Schubert sent the symphony to a friend, Anselm Huttenbrenner. It was a gift to the Musical Society of Graz for making him an honorary member. It was never played, for Huttenbrenner locked it up and kept it out of sight until 1865, 37 years after Schubert's death. Nine fully-scored bars of the scherzo exist. It has been argued that the symphony was finished and that this opening page of the scherzo proves as much, for it shows that the fragment breaks off on a left-hand page at the end of a book of double-sheet pages, on which the next book should follow with a top sheet forming the next right-hand page. How unlikely is it that a composer would write nine bars in finished full score and then suddenly stop. Also, at the end of the stave, in the last bar of the oboe part, there is a tie over a note with a slur. That slur shows that the note was to be immediately continued in the next bar, which means in this case, onto the next page. But there is no next page. Schubert wouldn't leave a piece in mid-air like that and post it off and forget it? Even I wouldn't do that :)
I reckon Schubert sent the whole symphony and that Huttenbrenner lost the second half of it. That would account for his silence and for quietly locking away what was left of it in the hope that nobody ever mentioned it again, and nobody did – Schubert because he was too busy composing – and remember he didn't own it, anyway, because he had given it away as a gift. I guess he assumed it was safe and complete in Graz. That's what I would have thought if I were him.