Rear parking sensors query

[mberry77]

Hi. I have just bought a 2014 Insignia Sri nav hatchback. It has the 8" sat nav screen with a rear parking camera.  The car has rear parking sensors on the back that don't work, I assumed they were factory installed but the colour is a slightly different shade of silver. I also found some cables trailing in the boot by the spare tyre so looks they may be after market ones. I get no warnings if any errors with them after I start the car. 

I thought the rear sensors came as standard, doesn't anyone know?