It is SPECULATION! there is a key pad on the passenger side of the cockpit which the co-pilot could have used. After all they have to go to the Loo or meet passengers so it is usual for the plane to be on automatic pilot. Anything could have happened, the pilot could have fainted, died of a heart attack, they have ruled out decompression sadly.
Stop reading the news reports until the actual Inquest once the Black Boxes have been 'read'!