Though it is rare, a unit can actually legally do a Fall-back toward the opposite board edge. When it comes down to the nitty-gritty they are simply trying to escape that one unit, not your whole army. What's more likely is the unit may move parallel to its board edge, which is still legal.
But as rare as these scenarios are, actually being "trapped" is far more uncommon. The unit literally has to be unable to move in most cases, as it is possible they could roll really low. A unit could have enemy units completely around them, with no gaps to move between but may not be trapped. Say they are in the middle of a square 8" on all sides of enemy models. If they roll a 2 to Fall-back they are safe. I'm assuming they could move in one direction without ending up within 1" of an enemy model. So imagine it is one model in the exact middle. So 3" is safe, even 4" if you move to a corner. But if you get within 1" of an enemy model and still have distance to move, you are "trapped."
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."