"There seems to be constant myths about insurance companies that they can just not pay out whenever they feel like it."
I'm guessing you haven't made many insurance claims. It's not a myth at all, if they can get out of it they will. We live in a corporate world where money is king. There was a storm a couple of years ago that blew out my fence and caused a leak in the roof, they sent someone to fix it and made it worse, then sent them back again to fix it. They didn't pay out because it apparently wasn't on the Beaufort Scale payout limit, something I was not aware when taking out the policy as I had never made a claim of that type before.
Is the car unattended? Yes
Then the following applies "You must lock all doors and close all windows and sun-roofs"
Was the car locked at all times while unattended? No.
Therefore your claim is invalid.
We can argue this point as much as we want but it boils down to the simple fact that the car was unattended and unlocked. I don't make the rules but that is my interpretation and I'm pretty certain it would be the insurers if they could invalidate having to pay money out.