Hey all, sorry to drag up an old thread but I thought I should give some sort of conclusion to it just in case anyone else searches for an issue like this.
So after reading the replies I decided to leave the Red in the house under lights (a Sansi 36w grow light) till 3 new leaves came out (and were healthy) which took about 3 weeks then put her back outside. The issue didn't pop back up again on her but the pup she made in August had one spot on a later leaf though I just left them alone outside. Right now it's doing fine (no spots like that) upstairs for the winter, leaves maybe a week slower to completely unfurl but still has a new one already ready to come out.
I've had a different banana that I picked up from a nursery (only labeled houseplant but it looks like either a blood banana or a Cavendish) do the same thing in September, though it occurred inside under lights and not outside like on the Red.
So thinking it back over and everyone's comments it probably was just from adjusting to new light and temperature. I do get spider mites inside but with citrus trees, ivys and herbs available they seem to leave the bananas alone, though I try to keep on top of them with neem. And water stress I've noticed gives me black edges not this issue that stays that light leathery brown until the leaf dies off months later.
I guess it's just an occasional blemish I'll live with
