This morning the power on IM4 was about half of what it should be, even though all other readings of IMC ower were good. It turned out that the IM4 whitening gain was reduced by 6 db on Saturday night at about 9.15 LT (2014-11-16 05.15 UTC). We don't know why that happenened and if something else was also changed at the same time. For the moment being, I changed the gain back to the previous value.
This was my fault - apologies for not putting in the log. The IM4_trans quadrants were saturating on Saturday evening, during the time when the CARM offset was being scanned to lock the arms; when I saw this I reduced the gain on IM4_trans by 6dB to bring the signals away from the rails. (The IMC WFS were saturating too, but since they are in-loop I didn't adjust them.)