The financial records would give the definitive answer.
Did he move assets around in his last days?
With his kind of assets I would think that they would be in constant motion. Like other monied people it would then be easier to skim a little here a little there and palce it in numbered accounts somewhere or in a mattress.
Heck, a guy like Fossett probably has substantial assets put away where nobody could get to them. With his wealth, there’s no way that anyone else knew where everything was.
And if you really were serious about it, and as bright as he was, he might have been planning this for some time.
I am not saying I believe he did it, but I sure wouldn’t dismiss it out of hand.