I originally read about this on Mashable
Usually when the president travels somewhere, the trip is known about in advance and his arrival is highly anticipated. Even when the president travels overseas, these visits are typically public knowledge. Today, President Obama made a surprise visit to Afghanistan on the one year anniversary of Navy SEALS killing Osama bin Laden.
A journalist in Afghanistan reported having seen Air Force One and took to twitter. Soon, American journalists caught wind of this and started reporting of the unannounced visit. As Mashale noted, Josh Hirsch from Huffington Post was the first: