I don't know the algorithm behind the programming of the Air, but during my tests it sometimes retraced the path and others it flew a straight line.Hate to contradict this but when my AIR lost contact it tried to return in a straight line. Unfortunately it clipped a tree and failed to return. I was lucky that it was found about a week later. Had my phone number on a label inside the battery bay.
What the manual says is that the vision system creates a real-time map during flight.
I'm guessing that if this map shows an abstruction it then does a retrace, if it doesn't, then it flies in a straight line...