The Locked-Room Scenario

The locked-room scenario refers to the isolated island that the crime takes place on. One of the reason the case of Harriet Vanger is so interesting is that she was on a locked-down island at the time of her disappearance, which would inevitably seem that she must be on the island somewhere. When Harriet was reported missing the only exit to the island, the bridge was blocked off where no body could get through, so the mystery remained how could Harriet go missing in such a confined area? Henrik shares with Blomkvist how police have hunted the island for her body, but nothing has turned up. This concept is used in mystery and thriller novels sometimes. It's a creepy yet frustrating concept for the reader to deal with. It seems impossible that someone could get out of a room that is locked from the outside, or from the inside, or such as this story an secluded island with no escape goats. It seems particularly impossible for Harriet to escape from the island considering the bridge was closed off at the time. I assume if the bridge was open her escape would be noticed anyway due to the Vangers being the only ones on the island.