Meant to make a note of this earlier, but the Maintenance deluge took over.
H1 was locked this morning. Apparently it had been down, but went back to Nominal Low Noise (NLN) on its own!
While in NLN, received a Verbal Alarm at 8:14am (15:14utc) about the PI of Mode #25 was ringing up. I took the gain from +100 to -100, and this seemed to turn it around, BUT we did drop out of lock 5min later. Mentioned the alarm and addressing of the PI to Terra.
I've gone over the spectra during this time and Corey did turn the mode around in time; it was successfully damping down (and well below usual lockloss amplitudes) when we lost lock probably for unrelated reasons. Thanks TJ for the successful alert system and Corey for the quick response.
Mode rang up in the first place due to phase drift; I had set up guardian controlled filter changes to account for this but there was an error that put the SUS_PI node down between the two locks last night/this morning, so filter switch never happened. I've since fixed this.