That's what it was that was weird about Louise I just remembered - when Spencer walked in with that other girl, Louise started yabbering. She's usually one of these very in control type of people, and she seemed to lose control a little bit at that point, thereby giving herself away.
It's the same story as Caggie and Spencer - both of these girls don't want Spencer's attention when he gives it to them because they know he's a bit of a wally, but when they see him with another girl they can't stand it. Weird. What is it about this fellow that magnetises them so? Judging from the look of him when he was in that hotel room with Louise last series, perhaps they think they can borrow his bras?