"The alarm was raised by a member of the public about 11am."
As an electrician, who has installed several security systems... why wasn't the electric fence tied into an alarm system? It's not that hard, farmers even have it installed in case their fence gets knocked down by a falling tree branch. The last thing you want is a herd of cows or horses loose. You'd think the last thing a zoo would want is an escapee oranguatan, but obviously they don't care the few hundred dollars it would cost to secure an exhibit that probably cost over a hundred thousand dollars.