I saw it on TV, but don't know if possible. The thief had one of those handy/portable car starter that you easily plug in your car lighter outlet, and then you can now start your car if your battery is dead.

So what the thief did with this portable starter, he modified the design in such a way that it will generate an eletrical charge to a prong, so he used the prong to touch the body of the car, and it just simply fried the car alarm. After that, he can use the slim jim to open the car's door.

Is that really possible to fry the car alarm, by introducing high current/charge on the negative terminal of the car?

OT:
About that key left inside the car. I had that happened to me in the US too, with a rental car. I called the rental car company and then they told me to break the quartet glass of the rear door, and then I was told later, that they will just charged it to insurance... hehehe Ayos sa solusyon...