every now and again when I get in the car the idrive screen switches to the PDC display and one of the rear PDCs sensor displays/detects a close proximity object and shows upto red bar on the screen and a continuos tone.

whenever this happens it is always the same sensor location and there is nothing anywhere near the rear of the car.

i have checked the sensor and it is free from dirt, wax etc, as this is happening intermittently i'm thinking maybe a trip to the dealers at some point is needed to have a new sensor fitted.

anyone else seen this ?

 

The only time that happened to me there was dirt on the sensor.
In your case I would bet the sensor is defect and you need to get it replaced by the dealer.