As this lock progressed, the 100 to 200 Hz region was improving and the range hit 80 Mpc right after 8:30 UTC. This is despite some huge noise in the 30 to 50 Hz region that came up at the same time. Around 8:49, the detector lost lock. The first attachment is the range, and the second shows the two parts of the spectrum going in opposite directions. It's not clear why the lock was lost, although the PI summary page shows something in the 3-8 Hz band growing exponentially (blue trace, third plot). Maybe it's this line at 4735 Hz (fourth plot)? It gets bigger and grows ugly sidebands as time goes on. I don't see it identified on the PI MEDM page though (I looked for 11649 and 28033 Hz as likely aliases). Edit: Actually, this could be the 10th order EY violin mode; see alog 19608. Is it possible something is going wrong with the damping and it's getting out of control? Or maybe it's nothing to worry about?
I was able to damp it with 0.1 Hz wide butterwoth +100dB filter and a gain of +0.001. And I was able to blow it up by flipping the sign of the gain. As Evan already mentioend in alog19612 this line indeed belongs to ETMY. Terra mentioned that this is not PI due to its non-exponential growth.