My bid is that the plane ended up in Western China, which is Muslim and has been very restive (as if the two don’t go together, LOL).
It’s there and the passengers are ok, but China is embarrassed by it and is keeping quiet.
My additon to the way-out-there but remotely possible theories: The passengers have been exposed to some sort of horrible disease, and they're isolated through the incubation period. The pilots are long gone.