Front Detector Operations

[StephenDaniels]

I drive a '13 plate XF Portfolio. I've had a front parking detector replaced as the old one had apparently failed. It had been temperamental for some time. It works 'automatically' when going from reverse into drive and in drive when the overhead 'p' button pressed.
I seem to recollect (but perhaps wrongly) that it originally worked 'continuously'.
Am I wrong about that. If not, what do I have to do to get it into this mode ??