Well, a couple more steps of diagnostics.
the sensor inputs of the BOB are free hanging around 8-9V when NOTHING is connected. I tested each limit with a simple wire to GND. XYZ worked fine reading all the way to Mach3. So, system works.
The prox sensors were being powered by the 8-9V sensor voltage backfeeding through the inputs. The sensors are supposed to be powered by "source" power. They were getting their only voltage through the signal wire. Their power was connected to the 5V on the BOB which was putting out nothing. So, tonight, move power source to the 24V source. See if they work now.
This "being powered through the signal" problem explains why if it was triggered by a proximity to metal, the other 2 sensors would drop their voltage too.
Worst case: new BOB;
Best case: 5V is bad and have to power them from another source.
Either way, I'm closer now than ever before.