Looking down, Stones sees Fu Manchu fiddling with the furnace door. They were too far away to see what he was using, but it was clear that Fu Manchu was messing with the lock.
And then, all of a sudden, the door swings open. In a panic the zookeepers dart down the hill and are able to stop Fu before he escapes again.
Though it was clear that Fu Manchu was responsible, they still had to figure out how he picked the lock.