Easy way to tell if the magnetic switch is on the seat, apart from looking that is, arm the alarm, if you hear two beeps it's on, if you hear two followed by a third deeper tone, it's been left off. The reason you can wheel yours around without the alarm going off is because when the alarm auto-arms itself after 30 seconds, the motion sensor is ommited. This is so it's not going off all the time when you stop to talk to your mates or fill up with fuel, to get the motion sensor on you have to arm it with the button. If it has already auto-armed itself, press the button to dissarm then press again to rearm with the motion sensor on. If you have no magnetic switch, I fitted one to a friends SD from maplins for about £2.50.