What could have caused this?

Hi everyone - hope someone can help. I just received a set of prints (35mm NPS 160) back from my lab. About two thirds of the frames have red blobs / circles on the prints (they also appear on the negatives in cyan) and I am wondering what might have caused them.

I attach an example photo. The red circle always appears along a central vertical line across the print in the portrait dimension, but the actual location along this line varies.

I can't think it is a light leak on the camera as there is no effect on the edges on the frames. I have also tested the shutter for pin hole light leaks with a torch and it appears to be fine. Not all frames are affected and there seems to be no correlation between the length of time a frame was sitting in the camera before being shot.

I notice on the negatives that even before frame 1 on the unexposed leader there appears these marks.

Any thoughts or experience much appreciated. Thank you.

I'm guessing it's a lab issue, particularly if the negs were processed using a roller-processing minilab machine. If it was dip-and-dunk, then it's less likely a lab issue.

If this appears on every frame at about the same spot, I doubt if it's the processing. I had a similar problem a while back while trying out a cheap Russian knockoff of the old Leica. It turned out to be caused the seating of the lens, or rather by its lack of proper seating. Look for ways that a dim spot of unfocused light could have fallen on the film frame. Leaks and reflections would be my primary suspects.