After rewatching last night's rerun of "Not What It Looks Like", at the end when they apprehend one of the "Hollys", she's freaking out because if they don't let her go to deliver the stolen diamonds, she knows they'll kill her friend. Stella/Flack are determined to arrest her. It shows a close up of Lindsay's face as she's watching the girl freak out over the thought of someone killing her friend, and she looks very troubled. That's when she pipes up and offers to go undercover as "Beth" to return the stolen goods. Seeing the girl so traumatized at the thought of someone killing her friend obviously resonated with Lindsay, and I actually missed "her look" during that scene, the first time around, and it was evidently supposed to be very significant in the overall "secret past" scenario. And that made sense, Lindsay being troubled over seeing a girl so freaked out over the thought of someone killing her friend, just another piece of the puzzle that brought the old memories back, since she also lost some of her friends.