Either the security light is not powered from where you think it is - or the switch is faulty - or the light is haunted... An electric light needs electricity - if it's on it has electricity.
Try pulling fuses or throwing breakers on the consumer unit (fuseboard) one at a time - if one of the circuits doesn't make it go off then the power may be coming from a neighbour's house - garage or shed.
It's not battery powered is it?
Peccavi, May 2009
