After Brooke's conversation with Alec at the mall, she's sort of been dwelling on the poor boy's relationship problems. Yeah, it's not her place to get involved, but she also has trouble staying out of peoples' business sometimes. Especially when it involves people who could practically be family to her.
Plus, if her boyfriend's best bro is down in the dumps, he's going to complain about it endlessly and ask her to fix it anyhow. She's getting a jump on things, right?
So, at school, she keeps on the lookout for the spitfire that is Clara so that she might be able to get some more information about the situation.