My guess would be that after looking at photographs of the layout of the square (which we saw Emma looking at after she received the video of Lauren visiting the Beale house) that she realised Jane had given Lauren a false alibi. Jane said she saw Lauren from the Masood's window & she never entered the Beale house, but looking at maps of the square there's no way you can see the front door of the Beale's house from the Masood house. Also looking at the video Emma would have seen that Jane's car wasn't in the square so how could she have been at the Masood house when her car wasn't there, which would account for Emma's statement to Max when he put her in the car to go to hospital, she said 'It wasn't there'.