Some years ago a local dealership had a brand new Sierra Cosworth on the forecourt. Fitted with a Thatcham alarm/immobiliser, they thought it was unstealable. A few days later it had gone. A few weeks later Police found the shell of the car. Apparently the thieves had cut through the bonnet right above where the battery is and disconnected the battery, thus rendering the alarm useless, towed it away and stripped it completely at their leisure.
A friend of mine had his Ford Granada stolen once. A week later it was used as the getaway vehicle in a bank job in Leeds and when police found his vehicle, there was a body in the boot
He didn't get the car back until about 10 weeks later (still with dried blood in the boot too) Needless to say, he sold the car shortly afterwards.