electric arc

I suppose an electric spark took place when I removed the cassette from the camera. I heard a noisy snap when the film was taken off.

It's only once the film was developped and scanned that I realized that 3 exposures had a single white curby line, that I started considering it was an electrical arc.

I don't believe it is a bromide formation.

Film: an old Ilford FP4+ shot at speed box.
Camera: Nikon L35AF
Developper: HC-110 dilution G (1-119) 60 min at 20C
semi-stand dev.: 3 inversions every 20 min.
Steel tank.

Click to enlarge the first picture. The white curb line is encircled.

yes, low hunidity, and the action of winding (or more often, rewinding) film into the cassette can cuase this to happen. I think that major vendor cassettes have the felt light trap trweated with anti-static measures to discourage this.