As we paid some attention to the ETMX baffle diode (as we wanted to obtain some data for the IR scattering in X arm) we have found that the output of these go negative (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=16083), unlike all other PDs.

In the attached trend for the past 300 days, the last time a reasonable positive current was observed for EX baffle PDs (ch6 and 7 in the plot) was June 21 2014 during one of the BSC9 vents. When we peeked down the arm after that vent (pink vertical line), the sign was already wrong.

Daniel remembers that an amplifier box was replaced around that time, so we'll swap the box with a spare.