When I was testing outlets in detached garage, I was fixing, I ran into a similar problem. I had the two normal lights, plus a faint glow on the abnormal light, so I had all three. I checked connections, and after I was done, I had the two normal lights only. I remember that when I did find a poor 'grounded conductor ("neutral") connection under a wire nut. Regardless, it was improperly connected, and needed correction. Just a guess, tom