Why does my vp shunt set off the security detector alarms in the shops it does it everytime i walk through them?

Something metallic?: Something of a metallic nature (or at least some detectable material by the sensors) is contained in your shunt. It's as simple as that. You could ask the surgeon who put it in if they can tell you exactly what the shunt is made of so that you have that information on hand in case you need it for another detector outside the country or somewhere a little more obscure to help in a pinch