There were two periods of unexplained motion of ITMY during undisturbed overnight lock on Sep 23. Both ISIs, plus all levels of the suspension (including oplevs) saw it, but there's nothing evident in the HEPI sensors or the ground.

These events made two periods of large noise in DARM (first plot), a few minutes apart, and a few minutes before lockloss due to PI. At that point, the PI didn't seem to be having any effect. The full Omega scan shows that ITMY has lots of anomalous motion. The second-stage ISI GS13s see a quick transient with rich structure (plots 2 and 3). The ST1 sensors see something similar but much less distinctly.

The L1 OSEM clearly sees a line below 1 Hz rung up for more than a minute afterward (plot 4).

I was not able to find anything on the ground sensors or HEPI that could have rung this up. I haven't looked as closely at the second event. I think this warrants more followup by someone familiar with the seismic isolation.