The trunk key cylinder itself is where the other lock is. For background - Volvo trunks essentially have two independent locks: The first is operated by the central locking system which activates in unison with the doors and fuel fill lid. The second is activated with the key cylinder in the trunk. One intent of this system is to allow you to lock-down the trunk, but still permit access to the vehicle interior with a valet key (which does not operate the trunk or glovebox cylinders). Also note that the fold-down rear seats can be hard locked from the trunk side.