The night between Saturday and Sunday there was a drift of the power on the dark fringe by about 15% (see Figure).
This took place over a period of 12 hours starting from one hour after the lock, so after the initial transient.
At the same time there is a drift of the LSC_SRCL_B1p signal (demodulation of the SRCL line in B1p_DC) i.e. the signal that measures how well the signal recycling cavity is locked on the top of the dark fringe.
At the same time there is a change in the alignment of the input beam wrt to the arm cavities seen by the B4 quadrants and a motion of the 56 MHz sidebands wrt to theĀ carrier seen by the B4 phase camera.