On cheap and expensive machines, zero position is usually determed by
a led-pt (phototransistor) pair or assembly.

I've disassembled the scanner completely. There's no place for an opto (if it
existed) to be "interrupted" (by a tab or such). There's nothing but a long
rectangular plastic holder that has a long pcb glued on the bottom and the
lamp and camera elements within. (Surprise! When I removed it and turned it
over, there was a label: "Canon". The flex cable connects to the pcb.

Used a digicam as you suggested. Nothing other than the exposure lamp.

It's still a puzzle how it registers ("zeros") let alone what's
malfunctioning.

Thanks.