If ammonia will not leak outside its self contained system, then there sho uld be no reason to have a detector and shut down system....but I am glad that there is both detector and shut down system. That said then depending on the source of the leak a considerable amount of ammonia could leak until the pressure is equalised. Operator training for unexpected events in any discipline is difficult so you can't rely on an operator remembering to drive outside when he gets an "ammonia alarm".
LPG, diesel and electric trucks do catch fire but are rarely driven outside the warehouse, the operator gets away as soon as possible, and to be fair in the UK employers do nor incourage or compel operators to take personal risks in this respect.