Moi aussi, but it's not entirely out of left field. The mob of pod people chasing them do enter the cave; one of them could easily have been carrying one of the lightweight pods, then stuck it somewhere out of our view. It's unclear just how long Miles is away from the cave, so the duplication process could have occurred during his absence. The real reason, though, that we're not shown any of that stuff is because otherwise the i-never-knew-the-meaning-of-horror-until-I-kissed-Becky business wouldn't have worked. We had to be as surprised as he is. In the novel, she escapes podification.