@Ry77
Hi Sorry to hear your hen is missing. It is not unheard of for sex links to go broody but it is unusual. How old is she? If she is just young still, then it is less likely. Do you know when was the last time she laid an egg in the nest box for you? When hens start to go broody, especially when they free range, they lay a clutch of eggs in a secret nest and then once they have enough, they start to incubate them. A clutch is usually 10-14. If you have had eggs from her in the last week then it is unlikely she is broody.
As stated by @nchls school , the other hens will probably not be aware of the location of her nest if she is broody. They can be quite sneaky about going back and forth to them. She will however come off the nest once a day to eat drink and poop. Your best chance of finding her is to keep a close eye and ear out for her returning to eat and perhaps drink. This will only take about 15 mins each day but may cause a bit of a commotion because broody hens can be a bit fractious and usually fluff their feathers up and make a regular cluck cluck almost like the ticking of a very loud clock, counting off the seconds till she has to get back to the nest. If you spot her then just observe and follow her back to the nest from a distance as she may not return to it if she thinks you are watching.... they can be quite sly.
If she is setting and you find her nest, you then have the dilemma of whether to mover her or not. If you move her, you will need to confine her to a cage within the coop so that she cannot return to the old site.... she is programmed to home in on the nest site not the eggs, so even if you remove the eggs she will probably still return to the nest. If you don't have a cockerel then you have the choice of breaking her of her broodiness and @aart has a good explanation of a broody buster cage or get her some fertile eggs. Letting her to continue being broody without hope of hatching eggs is not good for her health.
Good luck and I really hope for your sake that she is one of those exceptional sex links that goes broody.
Best wishes
Barbara