the card meter sends its data back to the company responsible for supplying your electricity each time you make a top up,I have seen this before several times and one guy told me that if they fiddled from day one there wasn,t much chance of being caught,it was only if the bill dropped dramatically that people came out to check.I E if you used say £10 a week from day one nothing was said but if you suddenly went from £30 a week to a tenner then someone would check.there was a daft scam years ago involving the old paper cards but it didn,t work,the trick was to cut a £10 card in half a certain way and you got £20 worth of electric,however when the meter was read the descrepancy was discovered and they got a bill.Most suppliers insist on reading a meter once a year,in the case of card meters this is more to check they have not been tampered with rather than taking an actuall reading.