I was thinking about this during Obama's recent tour through Asia. During that week Hang Seng and Kospi were plainly overvalued (Obama visited Shanghai, Beijing, and Seoul). Now, sure there may have been some speculation that there would be some announcements during the trip. But more than anything, I think the markets were buoyed by an ABSENCE of negative news during his visits.
Think about it - certainly the Chinese and Korean governments made sure to clear the table of any negative news during Obama's visit, in order to ensure all media coverage was upbeat and positive. Sure enough, there was a vacuum of bad news all week. There was a similar, though less pronounced effect when Hillary last came through the region.