Later in the episode, Spencer (Troian Bellisario) tells Mona, in full scary “A” mode, that Ali probably wasn’t watching someone, but instead was being watched by Mona. Spencer seemed pretty certain of this theory, but we’re not sure if she thought things through.
One argument in favor of the theory that Ali was, in fact, hiding from Mona in that scene. After all, we know Ali was getting threats from “A,” and that she had been making arrangements to meet “A” through the newspaper classifieds. Maybe Mona was in charge of this entire meeting, but just went into nerd mode to make Ali think she wasn’t “A,” and avert suspicion.
There’s one hole in that theory, however. Why would Alison ever team up with Mona to torture and hurt the Little Liars? Even if and when Ali found out that Mona was “A,” why would someone as confident and tough and ruthless as Alison decide to stop protecting her friends and help someone torment them?
Unless, of course, the twin theory is in play. If Mona or someone else killed Alison, then we could see Mona teaming up with the hypothetical Courtney DiLaurentis to torment the rest of the Liars. Is it possible Alison was trying to spy on her own twin when Mona approached her in that flashback scene?