Why would Garrett smile at her, or any of the Liars, when they were the ones who believed he killed Maya St. Germain (Bianca Lawson) all season long?
Here are a few possibilities.
Garrett is on the “A” team. We’ve had our suspicions about Garrett ever since day one, especially since he’s dated Jenna Marshall (Tammin Sursok) and Melissa Hastings (Torrey DeVitto), and he was with the NAT club in Alison’s (Sasha Pieterse) bedroom on the night she went missing. One theory is that Mona (Janel Parrish) left Radley Sanitarium for the day because she was going to bust Garrett out of jail. But once the “A” team found out about Nate/Lyndon’s dark plan to kidnap Paige (Lindsey Shaw) and threaten Emily, Mona had a “change of plans.” Instead of framing the Liars for Ali’s grave-robbery or sneaking Garrett out of jail, they could just blame it all on Nate. That way, Garrett would be released anyway. Therefore, Garrett would want to thank Emily for making it all happen.
By identifying Maya’s real killer, she freed him from jail. Even if Garrett isn’t on the “A” team, he would still want to thank Emily for outing Lyndon as Maya’s killer, since that evidence removes him as a suspect. He could be perfectly innocent.
Now that he’s free, the police won’t touch him again. When a town persecutes and locks up an innocent man, public sympathy is almost always on the guy’s side. If Rosewood police are too nervous to accuse Garrett of anything again, they’ll probably forget about that whole Page 5 incident (Jenna claimed that Garrett had told her to hold on to Page 5 of Alison’s autopsy report, even though they’d stolen it together.) If Garrett comes under suspicion again, they may choose to leave him alone instead of risking the same mistake. Not to mention, a lawsuit.