How old is this hen? Is it a new layer or an older hen?
 
If your girl was going broody, there are many more symptoms than simply sitting in a nest. Absent any of the other broody signs, missing feathers on the breast, a low, persistent "popping" cluck, an occasional outburst of screeching, all around bad mood, I'd say she may be having the urge to lay an egg, but it may be hung up.
 
I have had two hens lately, one age five, and the other age seven, exhibit difficulty in laying. They behaved much as you've described your hen, camping out in the nest box with no results. In both cases, after a twenty-minute warm tub soak, they each laid their egg. However, on a few occasions, the egg didn't appear until two days after the soak, but more often, the egg was laid soon after.
 
I also discovered that holding the hen on my lap on top of a warm heating pad for twenty minutes was just as effective as a warm water soak, and there was no soaking wet hen to deal with.