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Almost certainly magnetic. So just another reason to buy Audi ..They'd still have to closely chase in visual contact. Otherwise the moment the offenders learnt of these, when stopped at a junction/lights etc or out of sight in a side road, they'd just hop out and move it to an adjacent vehicle.
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In city chases at present they often have to take extreme avoidance risks when meeting traffic at lights etc. If the police are not right behind them but just tracking from a distance, they can be less desperate and also get chances to lose the tracker.The offenders are not likely to be stopping at lights and the delay of stopping the car getting out getting around the back then getting back in .. precious seconds lost.