We have a SENSOR CEM Dimming Rack with the 48 modules (96 circuits). It must be about 15 years old by now.
Recently I noticed that whenever our ETC ENCORE2 Console was shut down, a lamp on circuit, #80 comes on (full). I can do a CEM Reset to turn it off or it will go off by itself after about 20 minutes. One msg I saw said #80 "Not Set"! I replaced the 79/80 dimmer pack to no effect so I'm convinced its a CEM control problem. I even erased all Backup looks just in case somehow something had been set. We actually don't use this function and the clear all did not stop #80 turning on when the DMX signal disappeared. I'm not really sure of what I should/could do next. Ideas most welcome.
Take a phone over to the dimmers and call tech support.
My guess is that something is telling 80 to go to full, but the DMX loss behavior is cutting it off at 20 minutes. Backup won't run unless told to, but there is DMX loss behavior. There might also be something else in the system...
A short talk with tech support mentioned some old version of EOS could send random DMX during shutdown but to test that I ran black out and pulled the DMX cable out (I.E. No shutdown routine). After about 5 seconds 80 came on! So I like your idea of taking a phone to the dimmer rack. Will do so today.