Why is this the cold weather? After all, it's perfectly warm at my desk.
Because it's cold, I'm wearing a nice Aran jumper, as well as my usual polo shirt.
The jumper produces tiny amounts of lint.
One of those tiny pieces of lint worked its way into the recess through which the mouse light shines onto the underlying surface.
And those fine little fibres were causing the optical problems - because the lens could see them, and they weren't moving.
(Of course, I didn't look closely enough to see this until after I'd retrieved a new mouse from our support guy. I had to have time to consider possible failure modes in an optical mouse, and to come to the conclusion that even if the light appeared as bright as usual, something had to be affecting it.)
ETA: And now I check this one at home, it too had a little bit of clumped fibre in the same place.