The last one I heard about the windy was parked on a customers drive with the keys left in the ignition whilst he was working at the back of the house out of site from the van.
thats just asking for it.you might as well put a cone out with "please help yourself to my van". 
I don't think any of us purposely leave our keys in the ignition with the van unattended Daz. Unfortunately it can happen that you get distracted when you arrive at a customer triggering a simple change in routine which could mean a 'sliding doors' moment.
Recently the police were warning people to keep their front doors locked and never leave keys in the front door. Apparently there have been a spate of burglaries where someone has walked up to the front door, very quietly opened it slightly and felt for a key left in the lock. They remove the key and walk off. Often the home owner is unaware of this, locks up with another key and the rest doesn't need a script.
As there is visible no break and entry signs/damage you can be sure an insurance company will find a reason for making the claim null and void.
In the past we have had several letters from the insurance company advising that the van will not be covered by insurance if the back doors are left unlocked or open - so again they have thrown the 'foundation' clause which will likely be referred to in the advent of us making a claim.