Organised car thieves are not all stupid. When you think about it, music festivals are an obvious target. Park your car, leave it and don't return for a full day (or more), no security cameras, low paid car park attendant(s), disclaimer notice on the car park entrance (if any).....
Then you come back to find your posh car is missing some very expensive parts. If you saw someone messing with a car in a field full of cars - would you question them? To whom would you report it?
Very profitable, low risk theft - sad but true.
I reckon this type of theft is only going to increase.