This morning, after the IFO was locked, DARM was super noisy in kHz region. ISS error point was also super noisy and the coherence between the two was big.
Turns out that the ISS got noisy at the tail end of the lock stretch from last night, at around 2015-08-07 12:00:00 UTC. That's 5AM in the morning.
We went to the floor and sure enough a function generator was connected to ISS injection point via SR560. We switched them off, disconnected the cable from the ISS front panel, but left the equipment on the floor so the injection can be restarted later.
When Stefan and I hooked the injection back up, we found that the digital enable/disable switches weren't doing their jobs. Toggling the outputs of H1:PSL-ISS_TRANSFER2_INJ and H1:PSL-ISS_TRANSFER1_INJ had no effect on the appearance of the noise in DARM.