They always seem to be lit every time I see a voyager.
Possibly down to their age, exposure to the train side and the associated water and muck ingress. I have noticed one or two having some sort of sticky gunk on them reminiscent of Sellotape glue!
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Right, I managed to snap a pic but as yet I don't know how to upload. I can confirm however that there are two small panels side by side, one of them indeed is the water gauge which is the row of lights you described. The large red light alongside is the effluent tank level, this lights up once the level is over 50%.
They never used to light up until you touched the plate, yet th two parked up in Newcastle station just now had every one illuminated. Why this is I don't know. Perhaps an arriva modification I can only speculate.