Liberty system is a normal, modern, factory immobiliser system. All you really need is a coded key to steal the car. So if they steal your keys, they can steal your car. Subaru do an upgrade system with a bonnet switch, a siren, and sensors that can detect if a window is broken. Bit pricey though.
I would get a quality aftermarket system fitted if your worried. They can be activated by The factory remote locking/ unlocking the doors.
Just make sure it is fitted by a professional that knows what they're doing.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i