Could be using the CCD itself to detect a pattern underneath the lid
of the unit, just before the start of the glass sheet. That's how a
lot of them do their lamp check. Try looking for patterns or black
marks on the underside of the housing that surrounds the glass.

I thought that might be the case. Just beyond the clear glass area (in the
"home" end of the glass) is this pattern of white and black (view in
mono-space font). Area within lines is white:

__________ ___________ ___________
| |__________| |__________| |
|_________________________________________________ __|

I cleaned the glass. What else to do? If it's not "reading" this pattern,
could indeed be the reason its not "zeroing".

I'm guessing the ccd is toast. Or one of the traces on the ribbon cable (one
of those mylar PC affairs) broke.