I'm going to throw everyone for a "loop" no pun intended. If anyone paid attention, Young Joe slept with a hooker whom had a daughter named: SARAH. "Sarah" was the girl looking after Cid. Considering the hooker knew what Young Joe did, and that Old Joe went after the hookers daughter(Note long hair of the kid the hooker carrys to the room, it's blonde.) as one of the three targets this leads you to one conclusion. The hookers daughter is the Sarah watching the young Rainmaker. She herself looped back to change him from a young age, remember Cid stated that, "Sarah's a liar, she's not my mother. My mother was killed." and Sarah stated she was trying to raise him properly. Boom, now you know how Sarah know's what Looper's are.
I caught it the first round. If I hadn't heard the hooker state her daughter's name was Sarah, I would of never of caught it.