I agree also that an AFV moving out of a fortified building location would create a breach. I would always play it that way.
That being said, B23.711 which talks about an AFV creating a breach, while it only talks about traversing a hexside, it also references back to B23.41 when talking about the mechanics of the AFV breach. B23.41 only talks about *entering* a building hex.
If someone wanted to, there is an legalistic rules argument to be made that it has to happen upon entry only, and I would be hard pressed to point to a definitive rule citation to counter it.