Emperor knew Yoda was alive because they never found a body. You'd think the Emperor would want the most powerful Jedi's carcass right in front of him to ensure his victory but dude got arrogant and let it be. And the result of that was Yoda training Luke to get in Vader's ass. Had obi won really been the one that trained him, Luke would have died or turned sith. But because Luke got in that ass (thanks to Yoda), Vader was embarrassed and knew he would be dead so he through the Emperor in the bushes.