Are there symptoms? Is she panting, or is her comb purple? Is her abdomen swollen? She’s likely not egg bound, since hens with that condition won’t survive more than 48 hours. She’s most likely broody, but it‘s still good to be sure.
What happens when you pull her out? Making clucking noises and jumping back in box? Rest of the flock annoyed with her? Feathers puffed out? Likely broody and so off to jail if you don't need baby chicks. I have a jail/hospital but I tried a new tactic the past 2 times. Mine free-range and I have a very large run. So I keep the rest of the flock in the run so they can lay but I toss the broody outside the run. She obviously can eat and enjoy life but can't get in a nest box. Then at lock up I place her on a roost bar. Takes about 2 days busting broody.