A mobile data processing device configured to assist in locating a user. The device comprises an enclosure and an attachment base for attaching the device to a garment, lanyard or strap wearable by a user. The attachment base moves between a first (“normal”) position and a second (“alert”) position with respect to the enclosure in response to an external force acting on the enclosure. The device further comprises a detector system for detecting removal of the attachment base from the normal position, and an alert signal generator configured to send an alert signal in response to said detection. The alert signal can be generated quickly by grabbing the enclosure and pulling the device downwards.