@Jen-Brd
Hi
Is she in a pen on her own or in with other hens. If the latter, my guess would be that she is either being pushed out of the nest by other more dominant hens who want to lay in it or they are getting into her nest to lay whilst she is having a broody break and when she comes back she can't get into her own nest so she clambers into another one, perhaps even to wait, but then goes into her broody trance and is unaware that the eggs aren't under her. This is a problem with hens trying to brood in communal nests, especially if one nest looks much like another.
There is a chance that the eggs (if fertile) will still hatch but it may be delayed but if this continues to happen there is a good chance they will not make it. Can you place a board across her nest, so that she can't get out or other hens get in. The once a day, remove the board whilst you are doing daily chores and encourage her to come out for a broody break. She should be no longer than 15-30mins. Once she goes back to the nest, fasten her in and she will be safe on the eggs for the rest of the day. Broody hens prefer a dark quiet, safe place, so you would be doing her a favour in preventing the other hens intruding in her nest. If you are worried that she will not get enough food, you can always put a little pot of wet mash or even some scrambled egg in there for her, so that she has plenty, but they are quite capable of getting what they need on that short daily broody break.
If she is in a pen on her own, block off the other nest boxes, so there is only one to choose from, but I never heard of a broody hen selecting an empty nest over one with eggs so I'm pretty confident she is in a communal nest box and her companions are the reason for her playing musical nest boxes.
Good luck with resolving the problem and I hope they both hatch.....if you want more reassurance about the eggs viability, Casportpony has a thread about her unintentional and thankfully failed attempt to kill some hatching eggs, which involved a broody hen incubating them for a week and then them being removed and accidentally left on the kitchen counter for a week before being put into an incubator and they still hatched!!
Best wishes
Barbara