Of the many different alternative theories, the idea that she wandered from the apartment then was abducted/killed/died isn't beyond the realms of credibility. But then you still have to consider that she might have had a decent sense of direction as to where her parents would be although being 3 (almost 4) I might be wrong on that. I think the fact that the door was unlocked makes the intruder theory the likeliest. I'm doubtful that Tanner saw her being carried away though, I'd guess she was put straight into a car.