There are a lot of things we don't know, including the texts Hannah got from her friends, but we know she said she was going home, and she texted that she was lost.
I've been trying to make sense of the things we do know, and the video of Hannah walking all over seeming to be on a quest.
I have a theory that might fit what we know so far.
Hannah was dabbling in recreational drugs, but her friends did not know. Friendly Neighborhood Dreadlocks guy is a dealer. Hannah knows this, and may have bought from him in the past, so while she doesn't really know him, she kind of knows him.
Hannah leaves the party telling friends she's going home, but secretly wants to visit Friendly Neighborhood Druglocks guy first. She's a bit drunk, but knows his usual hang out spots. Thats's why she walks all over, she's looking for him. Maybe he hangs out at McGrady's Pub so she stops there. She looks on the patio but he's not there. She tries to go inside but is turned away because she's underage. She keeps on her quest, sobering up along the way. She realizes it's getting late and she has to get home so she appears a little frantic.
She heads to the mall, another hang out of Friendly Neighborhood Druglocks guy and begins to walk from one end to the other moving fast. Eventually she meets up with him, somewhere off camera. They enter Tempo, he has a drink, maybe she uses the bathroom. She tells him what she wants, and that she's been looking all over for him and has to get home, her friends are asking where she is. He tells her he doesn't keep it on him, so he can't get busted, but he has it in his car if she wants to walk over there with him. She thinks that sounds logical, so she goes with him. They walk to the car fast, because she's in a rush to get home. Maybe she tells him she told her friends she was lost to buy herself some time and an excuse as to where she was. Or maybe he suggests that she tell her friends that to use as an excuse for the time she was gone.
They get to the car and he seizes upon an opportunity. She has money, a phone and jewelry, and is young and attractive. Her friends think she's lost and might not expect her home soon. Maybe he cold *advertiser censored* her and puts her in the car and drives away. What happened after that I don't even want to think about.
I'm just speculating, trying to connect the dots. Maybe FND guy told her he was all out but had more at his house, if she wants he can take her there and then drive her home since she's in a hurry, so she got in the car with him.
I don't know, obviously, but it seems more believable than Hannah aimlessly wandering around (at a fast pace) and then leaving the mall with a complete stranger who has his arm on her waist or shoulder.