I've had air bubbles under fondant, and bubbles on the sides of buttercream cakes where the cake layers adjoin, but never one on the top of the cake.
The cakes were in the refrigerator until 3:00 and delivered to the party at 4:00 they were fine until around 7:00 when one of the developed the bubble. Sorry, I don't have a clearer picture, but you can see the cake in the back. What could have caused this? The customer did say the bubble went down on it's own about an hour after it appeared.