I did some more poking around on the web. It turns out that this water heater has a reset button for the ECO sensor. Reports on it are extremely mixed, but I figured I'd go look at it.
Well, the button appears to be sort of stuck in my not-too-educated opinion. In my somewhat better educated opinion, I now know what happened when the guy came to look at the water heater last Thursday.
As I walked around into the laundry room to turn on the overhead fixtures there to get him some more light, he removed the plate on the bottom front of the water heater to expose (among other things) this button. He then pulled the two spade connectors off of the button and hot-wired them together with electrical tape, completely bypassing the reset button.
Because, you see, the problem with the upper tank sensor obviously had something to do with this completely unrelated button.
*thud* *thud* *thud*
Oh, and then he left the system hot-wired together when he left.