[blinkybaby]

The fact that Rachel was next in after the four people involved in the task, means she alone would have known it would have to have been either Luke and Steph or Mario and Lisa who were the secret couple as she was not called into the diary room to be a part of the task. This is what the poster meant by Big Brother giving the task away, as it was easy to work out by a process of elimination rather than just guess work.