He was scheduled to visit a hospital per this how many knew that?
http://www.boston.com/whitecoatnotes/2012/09/13/mass-general-doctor-who-spoke-with-ambassador-stevens-just-before-his-death-pledges-commitment-for-peace-libyan-colleague/yEjOeOE7dQksPv64SGsidM/story.html
Everyone knew he was coming including the tribes
http://www.nytimes.com/2012/09/14/opinion/what-libya-lost-when-ambassador-stevens-died.html?pagewanted=all&_moc.semityn.www