Unfortunately, if somebody wants your car nowadays they will ask you ( not necessarily politely ) for your keys. They will do this by dragging you out of the car when you are stopped, maybe not voluntarily, maybe breaking a window at home at night and taking the keys if they can find them or otherwise asking you with a baseball bat ( other weapons are available ).
So forget the alarms and get a tracker type device. Which one will depend on the level of cover you want, some are monitored for movement of the car others wait for you to wake in the morning and find the car has gone - it will probably by then be on a boat out of Tilbury in a container.
Your choice ;)