More experimentation showed that a screwed on cover which is not totally closed works too. On a hunch, I measured the cover. It is made of metal. So, it is a conductor. So, it must be touching something.
Quick glance shows only a few flexible cables in the way.
My magnifying glass tells me that the cables are good. No break in the insulation. A look at the cover. Lets have a feel of the surface. Now it decides to bite me.