I posted here a few months ago about one on my parking sensors triggering erroneously.
Just in case this traps another owner, it turned out to be as follows:
I had fitted a TorkLift EcoHitch towbar (well, really a bike carrier mount).
Initially I just had the hitch and then, some weeks later, I added a tow ball. There were no initial problems with any of the parking sensors.
Sometime later I fitted a plastic/chrome towball cover, for when I didn't have the bike carrier attached.
About that time, ONE sensor started triggering. I swapped two of the sensors and the problem stayed at the same position - so, the sensor was actually OK.
Long story short - after some weeks we discovered that, even though the towball is dead centre, one of the parking sensors would detect it - but only when the plastic towball cover was in place.
Given the sequence I have laid out, above, I can't believe how long it took for us to discover the cause.