J. Driggers, J. Kissel

We don't have the fortitude to investigate further in this scary snafu, but we want to note it for future reference in case it happens again:
Once we arrive at nominal low noise, the SDF system informed us that ETMX's global yaw control output for the L2 stage only (H1:SUS-ETMX_L2_LOCK_OUTSW_Y) was OFF. This meant that there was no global yaw control (DOFs DHARD, DSOFT, CHARD, CSOFT) getting out to the PUM. 

We noticed it fast enough to 
    - set the ramp time to 4.0 seconds
    - ramp down the gain of the L2 LOCK Y bank from 1.0 to 0.0, 
    - turn on the output switch, as is nominal, and
    - ramp the L2 LOCK Y bank gain back to 1.0
in rapid succession such that we saw no noticeable effect on the IFO, and we were able to enter in to OBSERVATION READY unscathed.

I attach a trend of this channel, and it's similar channel for pitch over the course of O3B, since November. This switch has only been switched OFF 3 times (including today), and *other* than today, it happened while the IFO was down: 
    ISC_LOCK N State 11
    2019-11-12 18:35:25 UTC OFF Ghost
               18:38:37 UTC ON

    ISC_LOCK N State 12
    2019-11-26 20:35:57 UTC OFF Ghost 
               20:39:09 UTC ON

    ISC_LOCK N State 600
    2019-12-05 23:19:58 UTC OFF 
               23:51:27 UTC Jenne and Jeff turn it back ON

However, as one can see, these three times happen in three different ISC_LOCK guardian states, (11: READY, 12: LOCKING_ARMS_GREEN, and now in 600: NOMINAL_LOW_NOISE), and it's changing far LESS times that the ISC_LOCK guardian goes through the acquisition, implying that this switching has nothing to do with a rogue guardian.

We've also ruled out the ISC_LOCK guardian entirely, by perusing it for L2_LOCK_OUTSW toggles, and it's only in DOWN and DHARD_WFS that this switch is touched, and in both places, it's forced to 1.0 i.e. ON (maybe indicative of ghosts having spooked us in the past).