Quote Originally Posted by tkamiya View Post
I doubt it's an internal reflection....

You have a very well exposed and bright area - sky. Any reflection will need to overcome this brightness to show up. It's going have to be a one heck of a reflection to do that.

If you are in US and want to mail me a sample negative, I can put it under my microscope and see if I can see any surface contamination. PM me if you want to pursue this.
You are right, I did not think about it. Unfortunatly I live in Italy which is quite far from US and sending the negative would be unpractical. I have however access to microscopes and other stuff at work so I may try to get a look. Thanks anyway for your kind offer!