I have added a range finder to SITL, as described here. The rangefinder.distance value, which was not displayed before by QGroundControl is displayed now, but it is constant at 0, as is rangefinder.voltage.
Ah OK, I found out what was wrong. I have been floored again
SITL uses map terrain data to simulate the ground, and I just had placed my vehicle “beneath the floor” in the locations file. I just needed to climb higher than that.
After removing absolute-altitude,heading everything works as expected.