Witnessed (or at least witnessed the aftermath of):

Cell phones stolen, once by flight crew.
Passport stolen in flight. (It was stolen; not misplaced or discarded.)
Visa page in passport stolen. (Real stupid idiot that perpetrator was.)
Carry-on bag stolen a few times .... and it wasn't confused misidentification of luggage given the shape, color and uniqueness of the object on board the plane.
Laptop stolen in-flight.
Ipod stolen in-flight.

AFAIK from personal experience, it seems to happen way more than some stranger sexually assaulting someone while they are asleep.

I had my cell phone stolen in F. Taken while I was asleep. It was not someone getting lucky in finding it and keeping it.