A busy (and prolific) composer like Mozart was writing for the moment, not organizing his papers for posterity. So it's not surprising that a few things got lost or forgotten along the way.
The International Mozarteum Foundation announced last week that two previously undiscovered piano pieces by Mozart had been found. They're being closed-mouth about the details right now, but all will be revealed next week, when pianist Florian Birsak performs the pieces in Mozart's hometown of Salzburg.
Thanks to webmaster Michael Wells for the tip.