They can be pretty sneaky about getting out, I would check her nest to see if there is any poo in it... if she did mess in the nest then she may not have gotten up, but more likely she's just managed to do it without being seen. If you are worried about her condition then offer her a small bowl with tempting, high protein treats or change her food over the chick starter for the highter protein. With our 3 hens which are currently in different stages of their broodiness we got them off the nest every other day or so when we were out at the coop, just to make sure they were out and stretching. I honestly don't know if they would set till they die... though I imagine it would be a concern during weather extremes or if the hen's condition isn't great to begin with.