I was all set to post and then I thought hang on, the criminal types could be reading this too.
Needless to say that to nobble mine you will need more than the key or replacement ECU, that is all that I'll say.
The best way to protect the car from the 'opportunist' is the visible deterrent, against the determined / targetted theft there is no real protection when in public car parks etc.
When mine is home it is invisible from the road, generally parked behind another car and behind locked gates, the driveway it sits on has a perimeter alarm system, the garage it sits in front of is also connected to the house alarm and has 'proximity' alerts too.
The big one in our area is the knock on the door hijack so we don't answer the door to anyone we don't expect, we don't keep the keys for house or cars anywhere that they can be seen and we lock them all in a concealed safe when we go away for any length of time.
My next task is actually to install to drop / anti crash posts by the gates.