I currently have a broody barred rock. She's got a nest hidden somewhere. I was worried something had snatched her, but I started seeing her in the mornings when I'd throw some scratch and pellets out. I'd try to watch her, but eventually I have to get going to work. One of these days she'll slip up and I'll find that nest before the eggs hatch!
I had noticed her being aggressive towards any and all roosters, and then she disappeared for a few days before I started seeing her come out in the mornings all fluffed up and looking ticked off as she grabbed bites to eat.
I got her and two others at the same time, she's the only one that has gone broody. The others just lay their eggs and carry on.