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.