I am more concerned about some nut job tampering with my fuel by dumping in some extra ingredients, than theft. I have no data if its ever been done to a plane on the ramp when no ones around, but with 28 years on the job in LE, I can assure you there are people out there that do malicious things like that to automobiles, and other personal property, just for entertainment or vindictiveness. And we all know there are people that live near local airports that absolutely despise us pilots flying in and out making all that noise. Also, no one gets near a fuel filler cap with a hose unless its me.