It's simply the Sensor light at the top (the one you can see when the door's closed). It's a solid red light if there are no issues and a flashing red light to indicate that there's a problem. The manual calls it the Sensor Beacon.
We have an ETC Sensor CEM dimmer rack that has been flashing an error message for over a year, but nothing appears to be wrong (i.e. all the modules are dimming fine). I've cleaned out all the vents and gone through the trouble-shooting checklist and all seems to be well. Anyone have any ideas...