I have looked at why DET_MAIN arrived in the UNKNOWN state this morning

Figure 1 shows the transition from SHUTTER_CHECK_CLOSED to SHUTTER_CHECK_OPEN. This transition should have not happened because the slow shutter was closed properly. But it did happen because in the two seconds before the transition the power on B1p PD2 was very low, it was below the 0.002mW threshold, the Michelson fringes bring it up much above this threshold up to 0.02mW in the few seconds before and after, but during the time when the automation did the check the power is low. So DET_MAIN assumes it cannot check if the slow shutter is closed or not, because there is no laser power reaching SDB1.

There is a solution to mitigate this problem, when the power is low on B1p PD2, we can ask the automation to do the check of power several times before giving up and starting to check if the shutter is open. Currenly the automation tries once and then gives up.

Profiting of an unlock, the changes in the DET_MAIN node have been loaded in it at 15:32 UTC, and while in DOWN I requested SHUTTER_OPEN, then SHUTTER_CLOSED again, with no apparent issues during the transitions. The lock acquisition was then restarted.