Fresh from the IRB:
IRB Ruling #4 2008
A maul is formed with Team A pushing their opponents (Team B) back towards their own goal line with the ball being clearly visible at the rear of the maul, all the defending side (Team B) bound to the maul voluntarily exit the maul, has the maul successfully concluded or is the maul still active?
Law 17 Maul, Definition
A maul occurs when a player carrying the ball is held by one or more opponents, and one or more of the ball carrier’s team mates bind on the ball carrier. A maul therefore consists of at least three players, all on their feet; the ball carrier and one player from each team. All the players involved must be caught in or bound to the maul and must be on their feet and moving towards a goal line. Open play has ended.
The Designated Members have ruled the following in answer to the question
The maul has not successfully concluded and it is not still active. As the players of the team not in possession have all left the maul the maul ceases to exist and has not ended successfully or unsuccessfully as determined by the definition of a maul. The maul has ceased to exist and the ball is now in open play and the relevant Laws apply.