It may have been an abduction by a stranger, but it's also possible she went with someone willingly. I find it interesting that all her belongings were in the trunk, and the keys were not found. It's possible she put her stuff in the trunk for safekeeping and got in someone else's car to go for a drive. All she needed to take with her were the car keys.
If she was getting in her car to go home or somewhere else, why put everything in the trunk? I can see putting the backpack there, but cell phone and wallet, too? Especially the cell phone. And if some stranger abducted her would he/she take the time to put all her stuff in the trunk first?
Someone she knew could have been waiting, or pulled up in the parking lot, and said, "Hey get in. Let's go for a drive and talk." She puts her stuff in the trunk, takes her keys with her, and gets in. What happened after that is anyone's guess. But I think she knew the person and went willingly.
Since we don't know her habits, maybe it was her regular habit to put her stuff into the trunk of the car. She could have come out of B&N, put her stuff in the trunk, getting ready to enter the car, when somebody grabbed her. She looks to be tiny, so it would have been easy.
Number of young (and even not so young) females get abducted from parking lots and murdered by a stranger, Dru Sjodin one example.