I think the scooter is a good guess. Kamen's wheelchair is awesome, and the same technology applied to a moped-like device could be a very big business indeed. Anyplace you have a tropical or mediterranean climate, a fast quiet safe (at least compared to conventinal ones) scooter would sell in the millions of units per year. Which begs the question: how fast could he expand his factories?
The main argument against it being a scooter is that they would have to be road testing it. People have seen more unacknowledged (i.e. "black") military aircraft than they have seen of IT.