I lost lock at Oct 22 2018 01:51:16 UTC today, and afterwards the guardian asked that I "Check that fast shutter triggered on last lockloss".

I'm pretty sure it did. (see plot 2).  The fast shutter trigger goes off before the arm power decreases, according to the slow channels, and the OMC-DCPD SUM fast channel never exceeds 30 mA.  I took a quick OMC scan to make sure, and things seem okay.
Investigation
Prior to the lockloss, the FAST_SHUTTER guardian was in READY (state 21), but the LOCKLOSS_SHUTTER_CHECK guardian was in SHUTTER_FAIL (state 30).  Because LOCKLOSS_SHUTTER_CHECK was not in state LOW_ARM_POWER (aka state 1), ISC_LOCK told me to manually check the fast shutter.

LOCKLOSS_SHUTTER_CHECK entered SHUTTER_FAIL at GPS 1224202947, while ISC_LOCK was in ADJUST_POWER (state 506).  I had entered the ADJUST_POWER state to lower the power to 5 W because the violin modes had rung up too much, and INCREASE_POWER had caused OMCDCPD saturations at around 10 W.  I then moved on to NOMINAL_LOW_NOISE after the violins had calmed down, not noticing any notifications about the SHUTTER_FAIL state being entered.

In the CHECK_SHUTTER state, LOCKLOSS_SHUTTER_CHECK looks at the HAM6 GS13 channel and if the value is too low, returns SHUTTER_FAIL.  Here is an excerpt from the LOCKLOSS_SHUTTER_CHECK guardian log when it entered SHUTTER_FAIL:

2018-10-22_00:22:04.082186Z LOCKLOSS_SHUTTER_CHECK [CHECK_SHUTTER.run] USERMSG 0: run shutter check, then manually take to low power
2018-10-22_00:22:09.044349Z LOCKLOSS_SHUTTER_CHECK [CHECK_SHUTTER.run] timer['datawait'] done
2018-10-22_00:22:09.120162Z fetching 1224202932...
...
2018-10-22_00:22:09.122571Z fetching 1224202943...
2018-10-22_00:22:09.237937Z LOCKLOSS_SHUTTER_CHECK [CHECK_SHUTTER.run] No kick, but should have. Peak GS13 signal = 46.335
2018-10-22_00:22:09.302082Z LOCKLOSS_SHUTTER_CHECK JUMP target: SHUTTER_FAIL

This whole system check may have been messed up by the violin mode ringup and the increase and decrease in power.  

EDIT: We reached NOMINAL LOW NOISE again, all seems well.