Yep, an elephant, lion, bear and some other beasts can be very dangerous if they escape.
Unless they started off being there first and had a zoo permit, I don't think they would have the right to do that.
I can imagine the owner forgetting to lock the gate one night. I've done it with horses, dogs, and every animal I've owned - it's human nature. A loose lion would be scary though. I don't think the old shaking the feed bucket trick would work.