Light brown spots on the shell of duck egg during incubation

[UKDuck]

Hi, first time posting here.
I am on day 20 of incubation. Today during the misting and cooling I've noticed that 2 eggs have got strange pale brown spots on the shell, on the side. The egg on the picture is positioned pointed end down.
The embryo is happy moving inside, veins are still visible. What are these spots?

 

[My15Ducks]

I can’t see them in your picture very well. They could just be smudges of dirt if they’ve been there all along

 

[UKDuck]

The spots are very pale but still visible on the picture. Could they be stains from misting? Would be odd though, the water I use is boiled and cooled to lukewarm temperature.
The eggs were spotless and I wash my hands when handle them at all times.