Unfortunately the more "technical" explanation offered by Spanair didn't mention the exact nature of the indication that the pilot saw (i.e. whether it was a light indication of the device being on when it should've been off, a diagnostic light indicator of some malfunction with the device, the lack of light when there should've been one, etc). What was clearly stated is that both pilot and technician decided it was the heating device causing the problem/warning and it was disabled for the duration of the flight as it wasn't necessary.
I'm sure you're right about the announcement to PAX. I'm just curious to know what the pilot would actually see if the probe heater were to switch itself on while still on the ground. Does some kind of light come on in the flightdeck? Assuming that the operation of the heater is a 'silent' activity in normal conditions, ie not notified to the pilot, is there some system for detecting that it's operating when it shouldn't be?
I wonder if any MD-80 pilots / engineers could answer?
(I've spent years of my professional life untangling problems where the real cause is often nothing to do with the visible symptoms, and something about that warning light niggles me.)