Mark in view finder

Hi folks. On holiday in Sri Lanka with my new d7100. There is an annoying mark when I look through the view finder. It doesn't appear on photos, doesn't go away with a lens change. I have cleaned and cleaned the actual view finder, but it won't go away. I cannot see the mark during live view.

Any ideas where the mark may be and how/where I can clean it?

Thanks

It could be on the mirror.

Could be on the mirror or under the focusing screen but i would leave it untill you get back and try with a rocket blower brush,dont wipe them with anything its too easy to damage them.