Sounds like she might be broody. When you see her on a morning, is she acting strange? .... like fluffed up and clucking and defensive if any of the others come near. When you see her, watch where she goes after she has had something to eat and drink and she may lead you back to her nest of eggs. They usually only stay off the nest for 15-20 mins so it's not like you have to wait all day for her to go back to it. Of course I might be totally wrong and she could just be getting bullied and that's why she is not returning to the coop or she could be sick, but usually they stop eating if they are ill and stand somewhere hunched up.
Do you have a cockerel? They can and will go broody without one in the flock but obviously the eggs won't develop unless they have been mated. If you find a nest and you have a cockerel and you want to continue to hatch, then you need to decide whether to leave her in situ or move her and risk her abandoning the eggs. She will be homed in to that nest site rather than the eggs or nest so if you decide that it is not safe to leave her there, you will need to be careful about how you go about moving her or perhaps try to make her chosen location safer.
Good luck with figuring it out and let us know how things go.
Best wishes
Barbara
