Mystery solved:
It was the PPF that was installed without holes for the sensors. But that was not a problem initially and the story is interesting.
The "Mounted Object Detected" reared up some weeks after PPF was installed so I didn't associate the problem with that. And I washed it and saw nothing.
But, on the advice of
@borski,
@hydbob,
@EVCar, and
@Tâm, I looked with a magnifying glass. AHA
The PPF initially adhered to the sensor, and the sensor worked fine. Over time, as the PPF cured, it tightened over the slightly recessed sensor, thus detaching from it, and with just a fraction of a millimeter space between the sensor and PPF, the sensor reported that something was in front of it. It was undetectable except with magnification.
I used a razor and cut away the PPF and now all is great with the world.
Thanks for all the advice.