Mystery thin streak mid-frame

Retina IIa, Arista EDU 200

Some frames, not all show this thin streak. er.. actually two.

What happens?
Leak at camera back door, near a metal latch perhaps?
This was last taken at the permitted outdoor concert before MA restriction on gatherings, late afternoon.

Looks like a scratch, maybe.

I agree...emulsion scratch.
A light leak would be a DARK streak in the film (negative gets dense with more light striking it)

Not a light leak, which would show up dark on the negative.
Most likely a scratch, probably on the emulsion side as this looks like this it's gone all the way through to the film base. You'd be able to feel the scratch in the emulsion.
It could have occurred in camera, due to debris in the felt film trap of the 35mm canister, or on the rollers either side of the film gate. Probably not on the back door rollers or pressure plate as those only touch the base side of the film.
If you load the film yourself, could have happened at that time. Reusable cassettes are more likely to trap debris in the felt after being used a few times.
It could have also happened during processing. Especially if you wipe or squeegee the film before hanging to dry. I stopped wiping the film years ago. After a final rinse in Photoflo, I hang it to dry and don't touch it at all.

Definitely a scratch.

I took a quick look at the Retina film chamber, this morning.
The film advance shaft *may* have some minute scoring, possibly scratching film under tension, especially under stress during advancing motion.

I need to look the part under magnification, measure the defect's position in relation to what I have on film.

Arista EDU 200 / Foma 200 film had an issue with a bad batch of bulk rolls, but that has been about 6 years ago (unfortunately I purchased from that batch). Individually rolled films were not affected. Unless you are shooting old bulk film, that is not the issue. On the other hand, this film tends to be somewhat more scratch-prone than other films.

On the negative shown, there appear to be two parallel scratches, so I would look for two sources of camera irregularities a few millimeters apart.