A lot depends on when Adrian decided he was off to Red Bull. As is the way, this may have been signed and sealed, but not announced, months ago. And if that was the case. Then Im sure he would have visited RBR in secret and cast his eye over the aero team. He may even have made some "unofficial" comments over a glass of wine in that restaurant.
Its common knowledge that AN left Williams because they sacked Damon. I wouldnt be surprised to hear that Rons and his relations have become strained in recent years. Maybe as a direct result of David being released for JPM.
Off Topic slightly Adrians had some fantastic road cars in his time. A chipped up Sierra Cosworth and a Turbo'd Lotus Elise spring to mind.