It looks like yolk. How often do you collect eggs? Did you notice any yellow substance on the bedding?? If you have a egg eater they can consume it all and leave very little evidence. If you do have a egg eater you may find yolk on someone’s beak.
Thanks for the comments. I don't think that it is yolk. Several hours later, you really can't see much of anything on the egg. There is light shading, far less obvious. I would think that yolk would leave a more shiny residue after it dried. Only two are laying, and we check the nest box several times a day. No evidence of raw egg or shells in the nest box.
Would guess that it's just a bloom anomaly.
Tho spot at top looks to have some 'thickness' to it,
so could be a calcification anomaly.
Had that egg been washed in either of the photos?
If not, might be interesting to wash it(just with warm water and hands) to see how it feels while washing and how it look afterwards, both wet and after it dries.