spurious motion events from xinput

[Julian Bradfield]

Running Xubuntu 22.04 (and previous 20.04) on a Thinkpad X1 Yoga, I
have a long-standing problem where the screen never blanks. The
problem seems to start after the system has been up a while (I rarely
reboot). 

A bit of tracing shows that the X idle time is being reset every 30
seconds or so by a one pixel (or fraction of a pixel) motion event
from the Synaptics touchpad -- as seen at the xinput level.
However, evtest shows that there are no raw events from the touchpad.

What could be causing xinput to see (or generate) fake motion events?