My car recently started with a long beep from the rear when I turn ignition on and light flashing on pdc switch

Could be a failing sensor or wiring loom - probably water damaged.
I believe the wiring is tucked up in the rear bumper, could be worth checking for water ingress.
Also, check to make sure each sensor is 'clicking' when you put an ear to them whilst engaged, any that are not are the suspect ones. (I think that's the correct way around).  ON THE WAY: BMW F21 M140i
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Yeah, that's right.
Put your ear to each sensor (not while the cars moving tho )
If it's clicking, it's working.

Any that don't click can be replaced by removing rear bumper (not as bad as it sounds) and removing the sensor.
They just unclip from the back. Half hour job and sensors are around £10 on eBay.

Some that still click, can give false readings, so looking at error codes and live values will help as well.  Bodsys Brake Bible
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