Hi Paul. This may sound obvious, but when you indicate to pull in to the side of the road, turn off the engine, lock the car and leave it, if you didn't cancel the indicators before you left the car then both indicators front and back on either nearside or offside will stay on. This is a function on the vectra where if you were to leave the car parked on an unlit road at night you just leave the left or right indicators on when you leave the car, depending on which side of the road you are parked and hopefully stop someone clipping your car.
Michael Rawlings
April 2005