So a totally random piece of engineering feat was achieved yesterday.
IT snowed all day. - not uncommon for many at the moment.

Cleared the snow of about 10 inches of snow. Jumped in the car. Parking sensors made no long beep and the button wasn't flashing, reversed out of my space and it made the long continual high pitched beep as if blocked.
Brushed them clean. Drove home.
Jumped in the car this morning, reversed into a space at work, sensors worked as they should.

Happy that they work, puzzled as to why and how.

cheers for all the advice on this previously given 2019 RRS P400e Autobiography
2014 RRS 3.0 TDV6 HSE Dynamic
2006 RRS 2.7 TDV6 HSe
Alfa Romeo Guillietta 2.0 JTDM 2011
Mazda 3 1.6 2010
Subaru Impreza 2.0 WRX 2005

I have, or better the wife has , a TDV8, PDC sensors front and rear.
Similar problem, flashing indication on the dash and one longer beep, that's it.
Have checked the sensors if "ticking", none of them did.
Removed F36, engine running, to see if it was good which it was.
Upon re-inserting problem disappeared.
Rest of system like this although the fuse itself did show (very minor) signs of corrosion so possibly bad conductivity as well?
Anyway, works again.

Frank.