That's kind of the problem, it's nigh on impossible to replicate. If I try and exit the vehicle with the engine running, it warns me. If it's AS/S'd then it drops to off. I think there must just be something in their code that given a specific set of variables, doesn't fully deactivate it and allows the car to start again.
Let's put this into context, I have mild OCD and I'm of South African origin, so it may not surprise you to learn that I'm the type to make sure my car is properly locked before walking away. To the extent that I'll check by pulling the door handles (hence no comfort access option for me, that drives me nuts). I would definitely have heard the car running while I was performing my usual "I will be leaving my precious vehicle now, lets make sure it'll be here when I return" routine. I would definitely never leave a car running...
My current thinking (OCD kicking in and to try and validate this) is that I might get a little wireless video cam to put in the car with it set to alert on noise. Possibly an arduino with a motion sensor as it would certainly trigger on the diesel firing up.