The separation of both halves of the left engine fan cowl assembly due to improper latching and locking of the all the fan cowl latches. The cause of the improper latching and locking of the fan cowls was due to the failure of the mechanic to unstow and properly latch and lock the fan cowl latches after the engine maintenance had been completed, and the failure of the maintenance inspector to detect and identify the unlatched condition. Contributing the incident is the design of the fan cowl latch assembly that can provide a false latch condition when the latch is neither latched or locked. Also contributing incident is the lack of adequate Airbus and Jet Blue fan cowl latch inspection guidance to detect and identify an unlatched condition.