I've shot two rolls so far on my Zenit 12SD (1990). One was 400 tmax and the other was an Agfa 200.
On the first roll, maybe 2 of the 36 photos were affected by the light leak. In the second roll (color), maybe 3 images or so.
The first and last images were shot on a sunny day outside. In both cases I took a photo with the same settings shortly after I took the first, and there was no light leak (same shutter settings - 1/500). The middle photo was taken with a slow shutter (1/60 or 1/125) under low-light conditions (especially for 200 film) and was severely underexposed. The leak persisted across the photos taken after the middle photo.
The camera appears to be in full working order. The shutter curtains are clean. What could be the causes for the streak of light? Is it a light leak or flare or a scratch? If it's likely a light leak, how can I test for it?
If it's a light leak, is it likely produced during the winding process of the film back into the canister, or is it during the shooting?
Edit: Based on the answers, I went ahead and started to investigate:
I was also looking at the shutter fabric, and noticed that a part of it was slightly reflective which somewhat corresponds to the light streak print, do you think it's related? The fabric is reflective under flash and under a certain angle. It does not let light through. This I think is confirmed by the light streak going over the numbers on the negative film. This reflection appears when the camera has advanced the film, and the shutter is wound. What do you think?
I loaded a small flashlight inside of the camera and took it to a dark closet. After sitting in the dark closer for about 5 minutes, I started noticing a very small dot of light and line of light around the back, illustrated here in red (not my camera pictured):
The light was leaking out at the site of the backdoor hinge, and was very subtle. Here is the flashlight I used: