I do not have the answer but have experienced the same thing on only one plant. The possibility of your plant being a Happy Meal for insects is very real. Some insects live in the soil and only come out after the lights go off.Well she was fine last night I fed her and lights went out and came back on today and i found these!!! I know there is no bugs what i'm thinking its a type of nute burn where the leaf itself just burns away? Its only on 2 leaves at the top of the plant.