In cases where someone is known to have been abducted (such as Molly's case) the particulars of each individual situation point to what might be a more likely scenario than another. True, without a definite answer, anything is possible, but facts may point to one possibility being stronger than another.
If, for example, a child disappears from a large, busy shopping mall or carnival, it would be more likely that it was a random crime of opportunity by any "stranger" than a close relative or acquaintance who carefully planned the abduction. But - it does not rule anything out.
If someone disappears from a more secure or isolated place, where only a few persons would normally go, then the likely hood of the crime being perpetrated by someone having access or being familiar with the location is much stronger than a random occurrence. However, again, other possibilities still exist.
The motive of the perpetrator has to be considered, if one can be determined.