Here's my theory, as you call it.
From the people I've talked to (who don't even know each other) that are in the know in Plano/Allen area and the people that know her group of friends, it's apparent to me that she's a drug addict. I can't say whether she left the garage with the guy she was seen walking with or if she had someone else come meet her, maybe a dealer, but I'm pretty certain there was no abduction and there was no foul play.
Nobody wastes time and resources to search for someone without having an idea of what may have happened. Why search Allen? Well, they say because she's from Allen. Ok, why is that a good reason? I'll tell you why. They searched Allen because they know that she knows where to go and who to buy from, and if somebody met her at the garage, then they were probably an Allen resident, and she probably ended up in Allen.