Just a quick note in case these show up again:

During one of our locks this evening a number of lines showed up in DARM between 18 and 28 Hz (blue trace, 1225511648) and stuck around for ~half an hour. Their amplitude appeared very stable. We tried to track them down in LSC and ASC signals but didn't find anything convincing.

I turned off the violin mode damping, and the lines (gradually) went away (red trace), when I turned the violin mode damping back on the lines did not return. Not sure if this was because of the violin mode damping or something else. (note: the extra broadband noise in the red trace is due to the feed-forward tuning that was going on in parallel).