I think optimus will be absent for the majority the movie we may get a few scenes with him throughout the movie to see what happened with the creators but I think he wont return till right at the end. After they think they've beaten the bad guy. Optimus shows back up everyones all happy. hes back. they start running towards him to greet him like the trailer shows with bumblebee and Cade. Then optimus turns around and strikes them and im assuming a large battle follows and they realize oh shit this isnt optimus. Then unicron arrives and they all realize how wrong they were and whoever the bad guy was supposed to be was just a distraction by Unicron. And the Human bad guys are probably in on it to.