We have expanded the search of these drops' cause to more channels, and found plenty of witnesses (Fig.1). The channels we have found showing similar behaviour as the BNS range are:

• DC signals: B7, B8, B4
• 12MHz/112MHz sidebands on B4 and 112MHz sideband on B1p
• 56MHz sidebands on B1p/B4 phase cameras
• 50 and 56MHz quadrants on B1p, especially H/TY direction: the shapes are clearly visible on the Q components, while the alignment loops flatten them on the I. They can be found rather clearly on the marionettas' alignment though, both for WE and BS
• TY actuation on the input beam (IB_BENCH_TY)
• ASC_B2_QD2_14MHz, the channel previously used to identify jumps in the interferometer's working point

Among all these channels, no clear precursor can be identified. This behavior looks more like something affecting the whole interferometer at the same time, rather than a single loop misbehaving and affecting the rest of the machine.