[brianv]

My two-cents:

It looks like a light leak on the rear door. It's too linear to be a pinhole leak. It shows up on a blank frame as it's all-black, and anything will show up. Look for it to appear on the sprockets of other frames, normally black. If it is not running across the entire negative, into the sprockets, then it's probably something else.

"Leica it or not", This is another reason to use a camera in a fitted case. Keeps that film door tightly shut, and is another barrier to reduce leaks.