My mistake
I found a little black mold on some of my eggs. Most visible on the one in the picture on the bottom left.
I have been using the same white plastic milk bottle for filling up water bowls for some time. I also used it last to fill my egg box.
I left it a bit long before cleaning the bottle with diluted bleach.
When I did clean the water container it had a specks of black mold near the top.
A few weeks later the same was on an egg.
Lesson for me, always use clean water from a clean container for eggs and check the container more often.
They only have 3 weeks left so should be ok, but I will still remove the mold and rub a little athletes foot powder on the area just to be sure.