That is a MAJOR security breach. As a lead, I have worked RONs many times and it is my responsibility to do a full walk through the A/C to make sure no one is onboard before closing it up and pulling the jetbridge. OBVIOUSLY someone didn't check before they left for the night. Also, how is it that someone was able to jump the fence at an airport without any problem? Lots of people to blame here...from airport authority to the person responsible for securing that plane at the end of the night.
Live for Today.....tomorrow is NOT guaranteed.