Senator Hillary Clinton

ABC News' Eloise Harper and Rick Klein report:  It was one of Hillary Clinton’s most secretive missions of the campaign.

Suddenly, just days before perhaps the most anticipated round of voting, she disappeared from her plane, leaving aides to tell an angry and skeptical mob of reporters that Sen. Clinton, D-N.Y., went to "do something," assuring them that it was "nothing bad." Aides sat in the front of the plane, ignoring the people behind them for the two-hour flight from Texas to Ohio, Saturday afternoon.