usually when i find a hidden nest like that I've got one preparing to go broody, I have one that has only been laying not quite a month and she's already acting like she wants to brood but she is penned up and can't get out to go hide a nest. sometimes putting a box with a face on it and hole big enough for a hen to squeeze into helps, that way if they want to brood they know it's easily defended and feel safe doing it. The other hens may be doing it in support of her to protect her. Hens that go broody don't care if they are fertilized or not is one of the problems. if you can lock them in I would do so until all eggs have been laid in the coop to retrain them, it will take several days of doing this, do not let them out until all have laid should help break them of it.