This suggests that you've left your iOS device listed as 'Stolen' in your Tracking Control Panel.

Please refer to the article describing how Hidden for iOS tracks location: http://hiddenapp.com/support/faqs/how-does-hidden-ios-track-missing-device/

Setting your device as 'Not Stolen' in your Tracking Control Panel will stop the alarm notifications. If the alarm notifications persist after setting the device status to 'Not Stolen', try restarting the device.