This little ditty from Rick Jones and his terrific book, Classic Boxer Charging.
"A very faint glowing of the charge lamp (noticeable only on the darkest of nights), is fairly common on older machines and generally does not indicate an actual problem. This faint glow is usually the result of a voltage drop, in parts of the system, caused by an increased resistance, due to old age or poor connections."
Ricks continues to say that, "Very often, an ignition switch with loose or dirty internal contacts can cause a persistent dull glow". "If the light is glowing enough to be seen in the daylight (at road speed), then there probably is a charging problem. Most likely it will originate in the diodes".
Milo, didn't you recently change out your ignition switch? Did you refit a new or used unit?
I would recommend buying Rick's book to anyone tinkering with their Airhead. It's gold.