Smart Trolley

Another dull miserable day but I had to nip to the local Tesco this morning for a few essentials. A little intrigued by the trolleys - I know I can leave the "facility" but haven't been able to watch my left behind trolley to see what happens - but that's being a bit too literal. My inner yob wants to take the trolley and see how far it will go before applying the brake, but much too cold and wet to try today. Besides if it can do that it might send my GPS co-ordinates directly not just to Tesco-Central but also to Northern Constabulary HQ. The last thing I want is the Flying Squad appearing as if by magic. The trolleys look much the same as the dumb ones so it must be quite a neat piece of engineering. Perhaps there's some sort of sensor at the "facility" exits. Is there an onboard power unit? If any blipper knows how it's done please speak up. And if they're clever enough to engineer this why are some trolleys virtually unsteerable?