An add-on logging device (100, 300) mounted to a drug delivery device is turned on when the cap is removed. After a given amount of time in inactivity the sensor means of the add-on device is turned off automatically to save energy. If the user takes a dose of drug this is not detected as the add-on device is only turned on when the cap is removed. According to the present invention a warning message is provided when the cap is re-mounted after the sensor means has been turned off automatically, the warning message indicating to a user that an expelled dose may not have been detected.