Thanks all
To pick up some of the good points here, I've had a close look and the mark is definitely in the negative, not the scan. It appears to be in the image though, not a surface artifact.
I have used various things for drying including - nothing at all, gloved fingers, kitchen towel, chamois leather (disastrous BTW) and clean kitchen sponge.
I've uploaded another example which perhaps shows the effect more clearly, this time 120 film from a Lomo Diana F+. This example is very typical, a mostly straight line, sometimes segmented. I hope this might be a reference to others.
POSSIBLE ANSWER
It's funny how asking the question sometimes prompts you to come up with an answer yourself, even after weeks of pondering: I think I may have been wearing my Fitbit watch inside the changing bag! This watch periodically emits a green pulse of light which could illuminate the spirals and splash on to the film. Possibly this could explain why the effect isn't quite at the edge.
This is really stupid I know, even if it isn't the answer. I can only put it down to the over-excitement at home developing.
I'll post confirmation one way or the other when I next develop a roll, and thanks again for all suggestions.