Though we are not directly touching the BS (there's no actuator for it), based on some a theory that 2449.25Hz line (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=14855) might be the BS butterfly, I put a band stop ellip("BandStop",5,1,40,2440,2460) in FM2 of H1:SUS-BS_M3_ISCINF_L, and turned it on.

If we continue to have this line rung up, we know that it's not BS butterfly.