I wrote a response about the paper, but then re-read your statement and realised it was on the film. Perhaps some sort of drying mark? If not, perhaps your haunting films have become a bit haunted themselves!
I've been admiring your photos for some time now, but have neglected to say how much I enjoy them. This one in particular is both subtle and evocative, lovely and quiet. I'm sorry I can't be of any help with your little circle there (I was going with "physical imprint from the camera's red window", but then remembered that I know nothing about Arax 60s - does it even have a red window?!), even so, your photo is beautiful.
@awty I've had that same problem with codex rapid but it was the frame numbers that came through. I didn't even think about the marks on the backing paper with this one. Thanks!