Pseudo-broody?

Two of my black australorps have gone broody within the past couple months.
I've put fertile eggs under both -- one has a chick, the other is a week away from hatching.
Yesterday, I thought hen number 3 was broody -- she had all the signs:
--She was sitting in the nest when I went to gather the eggs, and she made that broody squawk.
--Her breast is completely bald.
All excited, I started hunting for the kind of fertile eggs I was going to put under her.
Then she hopped out of the nest and hasn't gone back.
Today, she's still wandering around the yard.
Is she broody or not?
Even though she has the classic signs, could it be something else?
I don't want to buy some fertile eggs and have her not sit on them.
Thanks.