I remember years ago, some people pulled a stunt of breaking into a BA
747 in LHR
, and making a video of them inside the 747...
Anyway, it can happen anywhere... they were caught at least. Probably thanks to Video Camera Surveillance. You can't stop people from breaking into an airport from the fence, but you must be able to see them and stop them as soon as you can.
Sydney for example, you can break in through many ways from the fence, and hop along to an aircraft... Even Singapore, there are ways to get in... LGW
too... There are many examples...
Security patrols, electrified wiring etc are just there for deterrence! Some airports test their security by actually hiring someone to break in and see how fast their security can get them!