A supplementary device is configured to be attached to a drug delivery device. The supplementary device comprises a non-contact sensor configured to output signals indicative of the position of a moveable component within the drug delivery device. The supplementary device further comprises a processor configured to receive the signals output from the non-contact sensor. This processor is additionally configured to determine, based on the signals, whether the drug delivery device is in a pre-activation state or a post-activation state.
